As filed with the Securities and Exchange Commission on  February 15, 2000

Registration No. 333-69649

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

AMENDMENT NO. 2

TO
FORM S-1
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

HYBRIDON, INC.
(Exact Name of Registrant as Specified in Its Charter)

Delaware                            2836                  04-3072298
(State or Other Jurisdiction of  (Primary Standard Industrial  (I.R.S. Employer
Incorporation or Organization)    Classification Code Number)  Identification 
Number)

155 Fortune Blvd.
Milford, Massachusetts 01757
(508) 482-7500
(Address, Including Zip Code, and Telephone Number,
Including Area Code, of Registrant's Principal Executive Offices)

SUDHIR AGRAWAL
President and Acting Chief Executive Officer
HYBRIDON, INC.
155 Fortune Blvd.
Milford, Massachusetts 01757
(508) 482-7500
(Name, Address Including Zip Code, and Telephone Number,
Including Area Code, of Agent For Service)

Copy to:
MONICA C. LORD, ESQ.
Kramer Levin Naftalis & Frankel LLP
919 Third Avenue
New York, New York 10022
(212) 715-9100

Approximate date of commencement of proposed sale to
the public: From time to time after this registration
statement becomes effective.

If any of the securities  being  registered on this Form are to be offered
on a delayed or continuous  basis  pursuant to Rule 415 under the Securities Act
of 1933, check the following box. |X|

If this Form is filed to register  additional  securities  for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list  the  Securities  Act  registration  statement  number  of the  earlier
effective registration statement for the same offering. |_|

If this Form is a  post-effective  amendment filed pursuant to Rule 462(c)
under the  Securities  Act,  check the following box and list the Securities Act
registration  statement number of the earlier effective  registration  statement
for the same offering. |_|

If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. |_|

CALCULATION OF REGISTRATION FEE
-------------------------- -------------- --------------- --------------- --------------- ------------- -----------
Title of Each Class     Amount to be      Proposed        Proposed       Amount of        Amount     Amount Due
of Securities to be      Registered       Maximum         Maximum       Registration    Previously
Registered                            Offering       Aggregate          Fee            Paid
Price Per     Offering Price
Share
-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Series A preferred            662,167       100 (2)        $66,216,700    $19,865.01     $20,063.59       $0
stock, $.01 par value

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.001        9,869,483    1.15625 (3)      11,411,589     $3,423.48      $3,571.82        $0
par value

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.001         460,000      0.54687 (4)       251,560          $76.23         $76.23        $0
par value

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common stock, $.001 par       173,333      3.00 (1)(6)       519,999         $157.58         $157.58       $0

value, issuable upon

exercise of Forum 
warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common stock, $.001 par     15,580,400         -(5)             -               -              -            -
value, issuable upon            (1)
conversion of  Series A
preferred stock

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common stock, $.001 par    3,002,958 (1)   4.25 (1)(6)      12,762,571      $3,867.05      $3,867.05        $0

value, issuable upon

exercise of Class A 
warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.001      1,752,945 (1)   2.40 (1)(6)      4,207,068       $1,274.74      $1,274.74        $0
par value, issuable upon
exercise at Class B 
warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.001       904,274 (1)    2.40 (1)(6)      2,170,257        $657.88        $657.88         $0
par value, issuable upon
exercise of Class C 
warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.001       672,267 (1)    2.40 (1)(6)      1,613,441        $488.87        $488.87         $0
par value, issuable upon
exercise of Class D 
warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.0001        609,195      2.40 (1)(6)      1,462,065        $443.01        $443.01         $0
par value, issuable upon
exercise of Forum  warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.0001        588,235      4.25 (1)(6)      2,499,999        $757.50        $757.50         $0
par value, issuable upon
exercise of Forum 
warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

Common  stock, $.0001       1,111,630     2.40 (1)(6)      2,667,912        $808.38        $808.38         $0
par value, issuable upon
exercise of Pillar
Investment  warrants

-------------------------- -------------- --------------- --------------- --------------- ------------- -----------

-2-

(1)      Pursuant to Rule 416, Hybridon is also registering that number of
additional shares of common stock that may become issuable pursuant to
applicable anti-dilution provisions.

(2)      Estimated solely for purposes of calculating the registration fee using
the proposed offering price of the Series A preferred stock, as
required by Rule 457(i). Does not include any shares of Series A
preferred stock that may be issued in the future as a dividend, which
shares are expressly excluded from this registration statement pursuant
to Rule 416(b) under the Securities Act.

(3)      Estimated solely for purposes of calculating the registration fee using
the average of the bid and ask price for the common stock on December
17, 1998, as required by Rule 457(c).

(4)      Estimated solely for purposes of calculating the registration fee using
the average of the bid and ask price for the common stock on June 29,
1999, as required by Rule 457(i).

(5)      Pursuant to Rule 457(i) no additional registration fee is required.

(6)      Estimated solely for purposes of calculating the Registration Fee using
the exercise price of the warrants, as required by Rule 457(g)(1).

The Registrant hereby amends this registration statement on such date
or dates as may be necessary to delay its effective date until the Registrant
shall file a further amendment which specifically states that this registration
statement shall thereafter become effective in accordance with Section 8(a) of
the Securities Act of 1933 or until the registration statement shall become
effective, on such date as the Commission, acting pursuant to said Section 8(a),
may determine.

-3-

The  information in this  prospectus is not complete and may be changed.  We may
not sell  these  securities  until the  registration  statement  filed  with the
Securities and Exchange Commission is effective. This prospectus is not an offer
to sell these  securities and is not soliciting an offer to buy these securities
in any state where the offer or sale is not permitted 

SUBJECT TO COMPLETION, FEBRUARY 15, 2000

HYBRIDON, INC.
155 Fortune Boulevard Milford, Massachusetts 01757

Secondary Offering Prospectus

662,167 Shares of Series A preferred stock

and

34,727,717 Shares of Common Stock

Hybridon is offering for resale by the holders in transactions
registered under the Securities Act of 1933 662,167 shares of Series A preferred
stock of Hybridon and 34,724,717 shares of common stock of Hybridon.

The common stock is quoted on the NASD Over-the-Counter or "OTC,"
Bulletin Board under the symbol "HYBN." Prior to this offering there has been no
public market for the convertible preferred stock.

See "Risk Factors" beginning page 5 of this Prospectus for a discussion
of certain factors that you should consider in evaluating an investment in the
Hybridon's common stock or Series A preferred stock.

Neither the Securities and Exchange Commission nor any state securities
commission has approved or disapproved of these securities or determined if this
prospectus is truthful or complete. Any representation to the contrary is a
criminal offense.

The date of this prospectus is __________, 2000.

TABLE OF CONTENTS

Page
Summary Financial Data........................................................3
Risk Factors..................................................................5
Forward-Looking Statements....................................................7
Business......................................................................7
Properties...................................................................18
Legal Proceedings............................................................19
Market for Registrant's Common Equity and Related Stockholder Matters .......19
Dividend Policy..............................................................19
Use of Proceeds..............................................................20
Selected Financial Data......................................................21
Management's Discussion and Analysis of Financial Condition and Results 
of Operations................................................................25
Directors and Executive Officers of Hybridon.................................32
Executive Compensation.......................................................33
Security Ownership of Certain Beneficial Owners and Management...............41
Certain Relationships and Related Transactions...............................46
Selling Stockholders.........................................................50
Description of Capital Stock.................................................54
Transfer Agent and Registrar.................................................56
Delaware Law and Certain Provisions of Hybridon's Restated Certificate 
of Incorporation, Bylaws and Indebtedness..................................56
Plan of Distribution.........................................................57
Legal Matters................................................................58
Experts......................................................................58
Additional Information.......................................................58
Index to Consolidated Financial Statements..................................F-1

-2-

SUMMARY FINANCIAL DATA

Years Ended
December 31,

-----------------------------------------------------
1996            1997            1998 
-----------------------------------------------------
(In thousands, except per share data)

Statement of Operations Data:
Revenues:
Product and service revenue............................            $1,080            $1,877           $3,254
Research and development...............................             1,419               945            1,100
Royalty income.........................................                62                48                -
Interest income......................................              1,447             1,079              148
-----------------------------------------------------
4,008             3,949            4,502
Total revenues
-----------------------------------------------------

Operating Expenses:
Research and development...............................            39,390            46,828          20,977
General and administrative.............................            11,347            11,026           6,573
Interest...............................................               124             4,536            2,932
Restructuring........................................                --            11,020              --
-----------------------------------------------------

Total operating expenses.............................            50,861            73,410          30,482
-----------------------------------------------------
Loss from operations......................................           (46,853)          (69,461)        (25,980)
-----------------------------------------------------
Extraordinary item: 
Gain on conversion of 9% convertible
subordinated notes payable..........................                --                --           8,877
-----------------------------------------------------
Net loss..................................................           (46,853)          (69,461)         (17,103)

Accretion of preferred stock dividend.....................                --                --            2,689 
-----------------------------------------------------
Net loss to common stockholders...........................          $(46,853)         $(69,461)        $(19,792) 
=====================================================
Basic and diluted net loss per common share from: 
Operations...........................................            $(10.24)        $(13.76)          $(2.19) 
Extraordinary gain...................................              --              --                0.75 
-----------------------------------------------------
Net loss per share...................................             (10.24)         (13.76)           (1.44) 
Accretion of preferred stock dividends...............              --              --               (0.23) 
-----------------------------------------------------
Net loss per share applicable to common
stockholders......................................            $(10.24)        $(13.76)         $(1.67)
=====================================================
Shares used in computing basic and
diluted net loss per Common Share (1)..................             4,576             5,050          11,859

Balance Sheet Data:                                                    Decembr 31,
-----------------------------------
1997              1998 
-----------------------------------
Cash, cash equivalents and short-term                                 $2,202            $5,607                
investments(2).........................................
Working capital deficit...................................           (24,100)           (5,614)               
Total assets..............................................            35,072            16,536                
Long-term debt and capital lease
obligations, net of current portion....................            3,282               473
9% convertible subordinated notes payable.................            50,000             1,306

Accumulated deficit.......................................          (218,655)         (238,448)               
Total stockholders' equity (deficit)......................           (46,048)            2,249                

(1)    Computed on the basis described in Notes 2(k) of Notes to Consolidated
Financial Statements appearing elsewhere in this Prospectus.

(2)    Short-term investments consisted of U.S. government securities with
maturities greater than three months but less than one year from the
purchase date.

-3-

SUMMARY FINANCIAL DATA

Nine Months Ended
September 30,
-----------------------------
1998           1999
-----------------------------
(unaudited)

Statement of Operations Data:
Revenues
Product and service revenue ........................................      $2,353       $4,644
Research and development ...........................................         950          450
Royalty income .....................................................          --          107
Interest income ....................................................         106           82
------------------------
3,409        5,823
Operating Expenses
Research and development ...........................................      17,181       10,106
General and administrative .........................................       5,818        2,947
Interest ...........................................................       2,880          562
------------------------

Total operating expenses .........................................      25,879       13,615
------------------------
Loss from operations ..................................................     (22,470)      (8,332)
Extraordinary item:
Gain on conversion of 9% convertible
subordinated notes payable ......................................       8,877           --
------------------------
Net loss ..............................................................     (13,593)      (8,332)

Accretion of preferred stock dividend .................................       1,647        3,194
------------------------
Net loss to common stockholders .......................................    $(15,240)    $(11,526)
Basic and diluted net loss per common share from:
Operations .......................................................      $(2.11)      $(0.54)
Extraordinary gain ...............................................        0.83           --
------------------------
Net loss per share ...............................................       (1.29)       (0.54)
Accretion of preferred stock dividends ...........................       (0.15)       (0.20)
------------------------
Net loss per share applicable to common
stockholders ..................................................      $(1.43)      $(0.74)

Shares used in computing basic and
diluted net loss per Common Share (1) ..............................      10,648       15,654

Balance Sheet Data: ...................................................    September 30,
1999
------------
(Unaudited)
Cash, cash equivalents and short-term investments(2)...................        $500
Working capital deficit ...............................................     (10,540)
Total assets ..........................................................       9,193
Long-term debt and capital lease ......................................         414
obligations, net of current portion
9% convertible subordinated notes payable .............................       1,306
Accumulated deficit ...................................................    (249,974)
Total stockholders' equity (deficit) ..................................      (4,507)

(1)    Computed on the basis described in Notes 2(k) of Notes to Consolidated
Financial Statements appearing elsewhere in this Prospectus.

(2)    Short-term investments consisted of U.S. government securities with
maturities greater than three months but less than one year from the
purchase date.

-4-

RISK FACTORS

Investing in our common stock is very risky, and you should be able to
bear the complete loss of your investment. You should carefully consider the
risks presented by the following factors, in addition to the other information
in this prospectus.

Our Financial Condition and Need for Substantial Additional Funding

If we do not secure additional funding by June, 2000, we will be forced to
cease doing business or file for bankruptcy.

If we do not secure additional funding by June, 2000, or if we do not
establish a suitable partnership or collaboration with a third party, we could
be forced to cease doing business or file for bankruptcy, and shareholders may
lose their entire investment. If this happens, you could lose your entire
investment. See "Management's Discussion and Analysis of Financial Condition and
Results of Operations."

Your investment could be substantially diluted if we issue shares to obtain
financing we need.

In order to obtain the funds we currently need to continue our
operations, and the additional funds we will need in the future, we may need to
issue shares of common stock or debt or equity securities convertible into
shares of common stock. We will probably need to issue a significant number of
shares in order to raise sufficient funds to pay our creditors, meet covenants
of our credit facility and continue our operations. This will result in
substantial dilution to your investment.

We are not in compliance with some of the covenants in our loan agreement and
note offering. If our lenders and noteholders foreclose, we will have few, or
no, assets to distribute to our shareholders.

We have taken out a $6 million loan and have completed a $7.1 million
8% note offering, both of which are secured by substantially all of our assets.
The loan and the 8% notes are owned in part by our affiliates. The loan
agreement for the $6 million loan requires us to maintain liquidity of
$2,000,000 and a net worth of $6,000,000. The 8% notes require us to maintain
liquidity of $1,500,000. We believe that we currently meet the liquidity
requirements, but we do not meet the net worth requirement. We do not expect to
be able to comply with these requirements in the future unless we are able to
obtain significant additional financing. Our lenders have in the past waived our
compliance with these requirements, but they may not be willing to do so in the
future. If our lenders and noteholders decline to give us waivers, we will be in
default and they will have the right to accelerate the repayment date on the
loan and the 8% notes and foreclose on our assets. Foreclosure will likely force
us to cease doing business or file for bankruptcy. If this happens, and we are
liquidated, there will be few or no assets available for distribution to our
shareholders. Since the debt is owned in part by our affiliates, the court may
treat the loan as a capital contribution or as a junior debt, in which case
there may be assets available for distribution to our shareholders, along with
the lenders.

We expect our operating losses to continue into the future.

As of September 30, 1999, we have incurred operating losses of
approximately $250 million. We expect to continue incurring operating losses
until revenues from the sale of any drugs that we succeed in developing exceed
our research and development and administrative costs. Assuming we are able to
obtain adequate funding to continue our operations, we will need to spend
substantial additional amounts on research and development, including
preclinical studies and clinical trials, in order to obtain the necessary
regulatory approvals. If we obtain regulatory approval, we will also need to
spend substantial amounts on sales and marketing efforts.
See "Business--Anticipated and Potential Costs."

Our Operations

We may not succeed in developing a commercially viable drug.

We do not currently have any drugs on the market and the drug
candidates we are working on are still in development. These drugs have not yet
been proven to be effective in humans. For example, our drug closest to

-5-

commercialization, GEM(R) 231, is still in Phase II clinical trials. All of our
other drugs candidates have not yet begun human testing. Historically, drug
candidates have a low overall probability of being commercialized, but that
probability increases as the drug progresses through the various development
stages. A drug may, for instance, be ineffective, have undesirable side effects,
or demonstrate other therapeutic characteristics that prevent or limit its
commercial use, or may prove too costly to produce in commercial quantities. If
we determine that our drug candidates cannot be successfully developed, or if we
are unable to obtain the necessary regulatory approval, we will not be able to
generate the revenues from the sale of drugs that we would need in order to be
profitable.

We have many competitors, and may not be able to compete successfully against
them.

Several companies, in particular Isis Pharmaceuticals, Inc. and Genta
Incorporated, are also in the business of developing antisense drugs. Isis has
received the approval of the U.S. Food and Drug Administration, or "FDA," for
Vitravene, is currently marketing this drug for the treatment of CMV retinitis,
and has several other drugs in clinical testing for the possible treatment of
cancer, including ISIS 3521 and 2503. Genta is testing G3139 in humans, also for
the treatment of cancer. These drugs candidates are further along in clinical
testing than Hybridon's cancer drug GEM(R) 231. Other companies have antisense
drugs in preclinical and clinical development, including Inex and AVI Biopharma.

In general, the human health care products industry is extremely
competitive. Many drugs are currently marketed for the treatment of cancer, such
as Taxol, Carboplatin, Taxotere and Camptosar. While it is unlikely that GEM(R)
231 will compete against these drugs, it may be used in combination with them.
GEM(R) 231 and other Hybridon antisense drugs may not, however, be able to
capture sufficient market share to be profitable.

Furthermore, biotechnology and related pharmaceutical technologies have
undergone rapid and significant change and we expect that the technologies
associated with biotechnology research and development will continue to develop
rapidly. Our prospects depend in large part on our ability to compete with these
technologies. Any compounds, drugs or processes that we develop may become
obsolete before we recover the expenses incurred in developing them.

Our ability to compete will suffer if we are unable to protect our patent rights
and trade secrets or if we infringe the proprietary rights of third parties.

Our success will depend to a large extent on our ability to obtain U.S.
and foreign patent protection for drug candidates and processes, preserve trade
secrets and operate without infringing the proprietary rights of third parties.

To obtain a patent on an invention, one must be the first to invent it
or the first to file a patent application for it. We cannot be sure that the
inventors of subject matter covered by patents and patent applications that we
own or license were the first to invent, or the first to file patent
applications for, those inventions. Furthermore , patents we own or license may
be challenged, infringed upon, invalidated, found to be unenforceable, or
circumvented by others, and our rights under any issued patents may not provide
sufficient protection against competing drugs or otherwise cover commercially
valuable drugs or processes. See "Business--Patents, Trade Secrets, and
Licenses."

We seek to protect trade secrets and other unpatented proprietary
information , in part by means of confidentiality agreements with our
collaborators, employees, and consultants. If any of these agreements is
breached, we may be without adequate remedies. Also, our trade secrets may
become known or be independently developed by competitors.

Our Securities

Because "penny stock" rules apply to trading in our common stock, you may find
it difficult to sell the shares you purchase in this offering.

Our common stock is a "penny stock," as it is not listed on an exchange
and trades at less than $5.00 a share. Broker-dealers who sell penny stocks must
provide purchasers of these stocks with a standardized risk-disclosure document
prepared by the SEC. It provides information about penny stocks and the nature
and level of risks involved in investing in the penny-stock market. A broker
must also give a purchaser, orally or in writing, bid and offer quotations and
information regarding broker and salesperson compensation, make a written

-6-

determination that the penny stock is a suitable investment for the purchaser,
and obtain the purchaser's written agreement to the purchase. The penny stock
rules may make it difficult for you to sell your shares of our stock. Because of
the rules, there is less trading in penny stocks. Also, many brokers choose not
to participate in penny stock transactions.

Certain existing stockholders hold a substantial portion of our stock, and
consequently could control most matters requiring approval by stockholders.

Our officers, directors and principal stockholders own or control more
than 60% of our common stock on a fully-diluted basis. As a result, these
stockholders, acting together, have the ability to control most matters
requiring approval by the stockholders. This concentration of ownership may have
the effect of delaying or preventing a change in control of Hybridon.

FORWARD-LOOKING STATEMENTS

This prospectus contains forward-looking statements that do not reflect
historical facts, but instead reflect Hybridon's current expectations, estimates
and projections regarding its business. Forward-looking statements can be found
in the material set forth under "Risk Factors," "Business," and "Management's
Discussion and Analysis of Financial Condition and Results of Operations," and
are characterized by use of words such as "believes," "plans," "expects," and
"anticipates." Forward-looking statements are not guarantees of future
performance, and necessarily involve risks and uncertainties, and Hybridon's
results could differ materially from those anticipated in the forward-looking
statements contained in this prospectus.

BUSINESS

HYBRIDON

Hybridon, established in 1989, is a leader in the discovery and
development of genetic drugs, which are drugs that treat diseases by acting on a
particular gene or protein. The genetic drugs being developed by Hybridon are
based on "antisense" technology, in that they use synthetic genetic material,
also called oligonucleotides, with the aim of stopping or reducing the body's
production of proteins that directly or indirectly cause a given disease.

Hybridon has developed and owns antisense technology that includes
important new medicinal chemistries (relating to the design and manufacture of
new medicinal compounds), analytical chemistry (relating to the detection and
identification of compounds inside and out of the body), and manufacturing
technology. It also has rights to technology allowing the chemical modification
of oligonucleotides, has particular expertise in the efficient design and
development of antisense drugs, and has devised innovations in the manufacture
of oligonucleotides. In addition, it has one of the few large-scale
oligonucleotide manufacturing facilities.

These aspects of Hybridon's business are discussed below.

TECHNOLOGY OVERVIEW

Introduction

The heart, brain, liver and other organs in the human body function
together to support life. Each microscopic cell within these organs produces
proteins that affect how that cell functions within its organ, and ultimately
how efficiently each organ functions within the body. Almost all human diseases
are caused by abnormal production or performance of proteins within individual
cells. In some instances, cell proteins act directly to cause or support a
disease. In other instances, cell proteins interfere with other proteins that
prevent or combat disease. Traditional drugs are designed to interact with
protein molecules that cause or support diseases. Antisense drugs are designed
to work at an earlier stage, in that they are designed to stop the production of
disease-causing or disease-supporting proteins.

The information that controls a cell's production of a specific protein
is contained in the gene relating to that protein. Each gene is made up of two
intertwined strands of DNA that form a structure called a "double helix." Each
strand of DNA consists of a string of individual DNA building blocks, called
nucleotides, arranged in a specific sequence. 

-7-

One of the paired strands  contains the information that directs the composition
of a specific protein,  and is called the "coding" strand. The other strand, the
"non-coding"  strand,   contains  a  different  but  complementary  sequence  of
nucleotides.  Each strand is made of linked molecules,  known as the "backbone,"
and attached to the backbone are molecules  known as "bases." It is the sequence
of bases that contains genetic information.

The full complement of human genes, known as the human "genome,"
consists of over 100,000 genes and contains the information required to produce
all human proteins. A copy of the complete human genome is present in each cell,
and each cell makes proteins based on its copy of the genome. Cells make
proteins in a two-stage process. First, the cell creates a molecule of messenger
RNA consisting of a string of nucleotides in a sequence complementary to the
sequence of the coding strand of DNA. This is called the "sense" sequence. A
sequence that is complementary to the sense sequence is called the "antisense"
sequence. The cell then produces proteins based on the information contained in
the messenger RNA. The number of copies of messenger RNA the cell produces will
affect how many copies of a given protein it produces.

A normal cell produces a given set of normal proteins in the right
amount for the body to function properly. A diseased cell produces inappropriate
or mutant proteins, or produces the wrong amount of normal proteins. A cell
produces mutant proteins when its DNA changes, either through mutation, as in
many types of cancer cells, or by infection with a virus.

Conventional Drugs

Most drugs are chemicals that stimulate or suppress the function of a
particular molecule, usually a protein, with tolerable side effects. Most side
effects arise when a drug interacts with proteins in addition to the target
protein. Generally, the fewer other proteins a drug interacts with, the fewer
the side effects.

Conventional drugs generally aim to bind only two or three points of
the target molecule. Frequently, however, sites on other non-target molecules
resemble the target binding site enough to permit the conventional drug to bind
to some degree to those non-target molecules. This lack of selectivity can
result in unwanted side effects, potentially leading to decreased effectiveness.

A further characteristic of conventional drugs is that developing them
is a time-consuming and expensive process, as for every compound that is found
to be effective and have tolerable side effects, thousands may be investigated
and rejected.

Antisense Drugs

An oligonucleotide with a sequence exactly complementary to that of the
messenger RNA of a specific gene can bind to and inactivate the messenger RNA,
thereby decreasing or eliminating the production of disease-causing or
disease-supporting proteins. Antisense technology involves the design and
synthesis of such oligonucleotides. Hybridon believes that drugs based on
antisense technology may be more effective, cause fewer side effects, and have a
greater range of applications than conventional drugs because antisense drugs
are designed to intervene in the production of proteins, rather than after the
proteins are made, and in a highly specific fashion.

Advances in mapping the human genome, including work conducted by
academic institutions, biotechnology companies and pharmaceutical companies,
have allowed many targets for antisense drugs to be identified. Once a gene
associated with a disease-associated protein is identified, an antisense
oligonucleotide can be designed, and the pharmaceutical effects of that
oligonucleotide can be improved by chemical modification. Chemically-modified
oligonucleotides can be composed of DNA, RNA, or a combination of the two.

Because the nucleotide sequence of a chemically-modified antisense
oligonucleotide is complementary to its target sequence on the messenger RNA of
a given gene, the antisense oligonucleotide forms a large number of bonds at the
target site, typically between 40 and 60. This allows it to form a strong bond
with the messenger RNA. A few identical messenger RNA molecules can cause the
cell to produce many copies of a protein; similarly, a few identical
chemically-modified antisense oligonucleotides can stop this process. This is
due in part to an enzyme called RNase H that can destroy messenger RNA bound to
an oligonucleotide without destroying the oligonucleotide itself, thus freeing
the oligonucleotide to bind with, and cause the destruction of, other messenger
RNA molecules. This process is generally

-8-

known as  catalytic  activity.  All of  Hybridon's  drugs are  designed  to take
advantage  of this  catalytic  activity  so that a  relatively  small  number of
antisense  molecules can effectively  inhibit  production of  disease-associated
proteins.

HYBRIDON ANTISENSE TECHNOLOGY

Hybridon's antisense chemistry builds on the pioneering work in the
antisense field begun in the 1970s by Dr. Paul C. Zamecnik, a founder,
consultant and director of Hybridon. The development of Hybridon's antisense
chemistry has been directed by Dr. Sudhir Agrawal, Hybridon's President, Acting
Chief Executive Officer and Chief Scientific Officer, and is based on what is
referred to in this prospectus as "advanced chemistries," namely Hybridon's
ability to alter the chemical makeup of the oligonucleotide backbone in a manner
that makes oligonucleotides safer and more stable without adversely affecting
their ability to promote the destruction of messenger RNA.

Medicinal Chemistries. Hybridon's first antisense drug, GEM(R) 91, was
based on first-generation phosphorothioate chemistry, which altered the
naturally-occurring, or native, form of oligonucleotides by replacing certain
phosphorus atoms in the backbone with sulfur atoms. GEM(R) 91 was more stable
than native DNA, but was still able to trigger the action of RNaseH, leading to
catalytic activity. However, there were side effects caused by the
administration of this modified DNA into the body. In particular, in the last
clinical trial of GEM(R) 91 three of the nine patients treated experienced
unacceptable decreases in platelet counts, which increased the possibility of
uncontrolled bleeding. As a result, Hybridon discontinued the GEM(R) 91 program.
Hybridon has, however, used the information gained from the human clinical
trials of GEM(R) 91 to design its advanced oligonucleotide chemistries.

Hybridon's scientists have designed and made over twenty families of
advanced oligonucleotide chemistries, including DNA/RNA combinations, also
called hybrid or mixed backbone chemistries. Hybridon believes that antisense
compounds based on these advanced chemistries will show favorable pharmaceutical
characteristics and significantly improve therapeutic value compared to earlier
antisense drug candidates. These compounds are likely to have the following
desirable characteristics:

o         fewer side effects

o        greater  stability  in the body,  thereby  permitting a patient to take
doses less frequently

o        greater potency, thereby permitting a patient to take lower doses

o        potential for multiple routes of administration  (such as by injection,
orally, or topically)

Manufacturing Technology. Hybridon's expertise in the synthesis of
chemically modified oligonucleotides has served as the foundation of its
manufacturing technology and know-how. Hybridon has developed proprietary
technology, including equipment, to increase the purity of its oligonucleotides,
make the production process more efficient, increase the scale of production,
and significantly reduce the cost of oligonucleotide-based drugs.

Proprietary Analytical Tools. Hybridon has established analytical tools
and processes that enable it to test the purity of oligonucleotides more quickly
and accurately than would be feasible using traditional methods. Hybridon uses
the resulting information to improve quality control, to assist it in complying
with regulatory requirements, and to monitor absorption and stability of its
drugs in preclinical and clinical trials.

Regulatory Know-How. Hybridon drug development and manufacturing
personnel have extensive experience in working with the FDA and other drug
regulatory agencies in an efficient and cost-effective manner. Hybridon often
assists customers of Hybridon Specialty Products, Hybridon's contract
manufacturing division, also called "HSP," by contributing essential components
of their submissions to the FDA.

DRUG DEVELOPMENT AND DISCOVERY 

The Drug Development and Approval Process

The process of taking a compound from the laboratory to human patients
generally takes 10 to 15 years. This process is extremely expensive and is
rigorously regulated by governmental agencies, including, in the U.S., the Food
and Drug Administration, or the "FDA". Each drug must undergo a series of trials
(preclinical and clinical) before the FDA will consider approving it for
commercial sale. The FDA or any company conducting drug trials can discontinue

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those trials at any time if it feels that patients are being exposed to an
unacceptable health risk or if there is not enough evidence that the drug is
effective. The FDA may also require a company to provide additional information
or conduct additional tests before it will permit a drug to proceed from one
phase of trials to the next.

The phases of preclinical and clinical trials are described below:

o        Preclinical Studies. Preclinical trials involve the testing of a given
compound in animals to provide data on the activity and safety of the
compound before the compound is administered to humans.

o        Investigational New Drug Application. If the data from research and
preclinical trials are promising, the company will file an
Investigational New Drug Application, or "IND," with the FDA. The IND
contains the results of the preclinical trials and the protocol for the
first clinical trial. The IND becomes active in 30 days unless the FDA
disapproves it or requires additional information. Once the IND becomes
active, the company can begin clinical trials in humans.

o        Phase I Clinical Trials. In Phase I trials, the drug is given to a
small group of healthy individuals or patients with the disease. These
trials are designed to produce data on the drug's safety, the maximum
safe dose, and how the drug is absorbed, distributed, metabolized and
excreted over time. In some cases, Phase I trials can give an early
indication of a drug's effectiveness. A limited Phase I trial is
sometimes called a Pilot Phase I trial.

o        Phase I/II Clinical Trials. In Phase I/II trials, the drug is given to
patients with the diseases to evaluate safety and to get an early
indication of a drug's effectiveness. This type of trial is commonly
used in the evaluation of oncology drugs.

o        Phase II Clinical Trials. In Phase II trials, the drug is given to a
larger group of patients with the disease for purposes of evaluating
the drug's effectiveness and side effects at varying doses and
schedules of administration and thereby determining the optimal dose
and schedule for the larger Phase III trials that follow.

o        Phase III Clinical Trials. These trials generally have a large number
of patients. The primary purpose of a Phase III trial is to confirm the
drug's effectiveness and produce additional information on side
effects.

o        New Drug Application. Once Phase III trials are complete, the company
will file a New Drug Application, or "NDA," with the FDA. The NDA
contains all of the information gathered from the Phase I, II and III
trials. Based on the FDA's review of the NDA, the FDA may approve the
drug for commercial sale. The FDA may deny an NDA if the applicable
regulatory requirements are not met. The FDA may also require
additional tests before approving an NDA. Even after approval by the
FDA, the company must file additional reports about the drug with the
FDA from time to time. The FDA may withdraw product approvals if a
company fails to comply with ongoing regulatory standards or if
problems occur after a company starts marketing a drug.

o        Accelerated Approval. The FDA is authorized to grant accelerated review
to NDAs for drugs that are intended to treat persons with debilitating
and life-threatening illnesses, especially if no satisfactory
alternatives are available. The more severe the disease, the more
likely it is that the drug will qualify for accelerated review. If a
new drug is approved after accelerated review, the FDA may require the
company that filed the NDA to conduct specific post-marketing studies
regarding the drug's safety, benefits and optimal use.

The regulatory process in other countries is generally similar to the
U.S. regulatory process.

Hybridon Drug Development and Discovery Programs

Hybridon is focusing its drug development and discovery efforts on
developing antisense compounds for the treatment of diseases in three major
therapeutic areas: cancer, viral infections and diseases of the eye.

Hybridon believes there are significant additional opportunities for
the use of antisense, particularly in the treatment of cancer. Compared to
conventional anti-cancer drugs, antisense may provide:

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o        more specific therapy

o        more rapid development of drugs targeting newly-discovered
cancer-related proteins

o        fewer toxic side effects, thereby allowing repeat and long-term
therapy, either alone or in combination with other cancer therapies
(such as radiation or chemotherapy)

o        when used in combination therapy, therapeutic effects that complement
the benefits of conventional drugs

For these reasons, Hybridon is exploring new antisense targets relevant
to the treatment of cancer.

CLINICAL PROGRAMS

Hybridon has conducted clinical trials with antisense drugs targeting
the following diseases. Hybridon is seeking partners for each of its compounds
in clinical development.

Cancer

Unlike normal human cells, cancer cells grow in an uncontrolled and
harmful manner. The protein molecule protein kinase A, or "PKA," has been
implicated in the formation and growth of various solid tumors, including colon,
ovarian, breast, and lung tumors. There are two kinds of PKA. It is normal to
find type I in developing fetuses, but abnormal to find it in adults. By
contrast, PKA type II is found in, and is necessary to the health of, normal
adults. Certain cancer cells produce PKA type I in adults. Hybridon has
developed a cancer drug, GEM(R) 231, that is designed to reduce the production
of the harmful PKA type I without interfering with the production of PKA type
II. Current drug candidates based on conventional mechanisms have unacceptable
side effects.

Hybridon has conducted a Phase I clinical trial to evaluate the safety
of GEM(R) 231 at multiple doses, and has found that patients tolerate it well.
This trial explored the maximum tolerated dose of GEM(R) 231 for both single
doses and multiple doses, and even high doses of GEM(R) 231 did not show the
side effects normally seen with current cancer treatments. This trial was not
conducted for the purpose of evaluating the efficiency of GEM(R) 231.

Hybridon is currently conducting additional studies with GEM(R) 231 in
patients with solid tumors that had not been cured by prior therapy. These
studies include a pilot Phase II trial and a Phase I/II trial. In addition,
Hybridon has begun the first in a series of Phase I/II trials treating patients
with solid tumors with GEM(R) 231 in combination with the anti-cancer therapies
Taxol(R) and Taxotere(R).

HIV-1 and AIDS

Acquired Immune Deficiency Syndrome, "AIDS," is caused by infection
with the Type 1 Human Immunodeficiency Virus, or "HIV-1," and leads to severe,
life-threatening impairment of the immune system. AIDS therapy using a
combination of drugs has resulted in decreased rates of death and improvement in
the quality of life for patients who are HIV-positive or have AIDS. There are
however, increasing reports that this therapy may be failing to give sustained
clinical benefit. Hybridon believes this underscores the need for new AIDS
therapies.

Hybridon has completed a Pilot Phase I clinical study in Europe of
GEM(R) 92, Hybridon's advanced chemistry compound for the treatment of HIV-1
infection and AIDS. This study was designed to explore the safety of GEM(R) 92
and to provide information on its absorption after oral dosing and injection.
The patients tolerated well all doses given in the pilot study. Further, GEM(R)
92 was detected in the blood after both oral dosing and injection, suggesting
that it may be possible to develop GEM(R) 92 as an oral drug. Hybridon believes
this was the first study of the oral administration of an antisense molecule to
humans. In in-vitro studies, beneficial effects were observed when GEM(R) 92 was
used in combination with several marketed AIDS drugs. Importantly, both its
medicinal approach and genetic target are unique, in that no antisense drug has
been approved for the treatment of AIDS, and no other drug has the same target
on the HIV-1 genome.

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PRECLINICAL PROGRAMS

Hybridon has also conducted preclinical studies in the following areas:

-------------------------------------------------    ------------------------------    --------------------------------------
Primary Therapeutic
Target                                               Indication(s)                     Status
-------------------------------------------------    ------------------------------    --------------------------------------

MDM2 (a protein involved in programmed cell          Cancer                            Ongoing; part of the Searle
death)                                                                                 collaboration

-------------------------------------------------    ------------------------------    --------------------------------------

Vascular Endothelial Growth                          Cancer                            Seeking partner
Factor (a protein that can cause abnormal            
formation of new blood vessels)                      Retinopathies (e.g. macular       Seeking partner
degeneration and diabetic
retinopathy)

Psoriasis                         Seeking partner

-------------------------------------------------    ------------------------------    --------------------------------------
Hepatitis C Virus                                    Hepatitis C (which can lead       Seeking partner
to liver cancer)
-------------------------------------------------    ------------------------------    --------------------------------------

HYBRIDON SPINOUTS

Hybridon has used multiple strategies to fund applications of its
antisense technology that it cannot develop at present without external funding.
Hybridon has used one such strategy, formation of spinout companies, to form
MethylGene, Inc. and OriGenix Technologies Inc. for the continued development of

certain product candidates.

MethylGene, Inc.

In 1996, Hybridon and three Canadian institutional investors formed
MethylGene. Hybridon owns approximately 30% of MethylGene. Hybridon has granted
exclusive worldwide licenses and sublicenses to MethylGene to develop and market
(1) antisense compounds to inhibit the protein DNA methyltransferase for the
treatment of any disease, (2) other methods of inhibiting DNA methyltransferase
for the treatment of any disease, and (3) antisense compounds to inhibit up to
two additional targets for the treatment of cancers. Research has shown that DNA
methyltransferase, a protein, is overproduced in some tumors, such as
non-small-cell lung cancer, colon cancer, and breast cancer tumors. MethylGene
is obligated to purchase from Hybridon at specified prices all bulk
oligonucleotides that MethylGene requires. Hybridon is also performing drug
development and other services for MethylGene.

The Canadian investors who invested in MethylGene have the right to
exchange all (but not less than all) of the shares of stock in MethylGene that
they initially purchased for shares of Hybridon common stock on the basis of
37.5 MethylGene shares (for which they paid approximately U.S. $56.25) for one
share of Hybridon common stock (subject to adjustment for stock splits, stock
dividends and the like). This option expires no later than 2001.

MethylGene commenced Phase I clinical trials of its first compound,
MG98, for the treatment of cancer in May 1999.

OriGenix Technologies Inc.

In January 1999, Hybridon and three Canadian institutional investors
formed OriGenix to develop and market drugs for the treatment of infectious
diseases, with an initial focus on viral diseases. Hybridon owns approximately
40% of OriGenix.

Hybridon has granted to OriGenix worldwide exclusive licenses and
sublicenses to antisense technology developed by Hybridon for the treatment of
human papillomavirus, or "HPV," and hepatitis B virus infections. HPV 

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infection can cause a variety of warts, including benign genital warts. HPV
infection can also lead to cervical cancer. Hepatitis B infections can lead to
liver cirrhosis and cancer of the liver. OriGenix may in the future negotiate
with Hybridon for licenses or sublicenses relating to additional targets. In
addition, OriGenix is obligated to purchase from Hybridon at specified prices
all bulk oligonucleotides it requires. Hybridon may also perform drug
development and other services for OriGenix.

CORPORATE COLLABORATIONS

An important part of Hybridon's business strategy is to enter into
research and development collaborations, licensing agreements, or other
strategic alliances with others, primarily biotechnology and pharmaceutical
corporations, to develop certain products. Hybridon intends to proceed with
Phase II clinical trials of its cancer drug GEM(R) 231. Otherwise, Hybridon does
not anticipate proceeding with any of its other clinical programs beyond their
current stages of development without a collaborative arrangement with a
corporate partner. Hybridon is currently a party to corporate collaborations
with Searle and Medtronic. Hybridon expects to retain the rights to manufacture
many of the products it may license pursuant to its existing and any future
collaborations.

G.D. Searle & Co.

In January 1996, Hybridon and Searle entered into a collaboration for
research and development of therapeutic antisense compounds in the areas of
inflammation/immunomodulation. The collaboration agreement, as modified in April
1998, provides that Searle may also select specific targets in the fields of
cancer and cardiovascular disease.

Hybridon and Searle are currently conducting research and development
relating to compounds targeting MDM2, a protein that is involved in programmed
cell death and that may play a role in cancer. In this project, Searle is
funding certain research and development efforts at Hybridon, and Searle and
Hybridon have committed personnel to the collaboration. The initial phase of
research and development activities will be conducted through the earlier of (1)
the achievement of certain milestones and (2) January 31, 2000, subject to early
termination by Searle. The parties may agree to extend this collaboration.

Searle has notified Hybridon that it intends to inform Hybridon by
February 29, 2000, whether it is electing to extend the collaboration.

In addition, Searle may designate up to six additional molecular
targets in the specified fields on terms substantially consistent with the terms
applicable to the initial targets. To do so, it must pay specified cash amounts
(in addition to specific research payments relating to each additional target)
and purchase additional common stock from Hybridon (at the then fair market
value), with the total payment per additional target equaling $10,000,000. If
Searle designates all of the additional targets, Searle will pay $24,000,000 in
cash and purchase $36,000,000 of equity. If Searle has not designated all of the
additional targets by the time any drug relating to the initial molecular target
reaches a certain stage of preclinical development, Searle must purchase up to
an additional $10,000,000 of common stock (at the then fair market value) in
order to keep its right to designate any of the additional targets. This payment
will be credited against the equity investment payments made by Searle for any
additional targets it designates in the future.

Searle has exclusive rights to commercialize any drugs resulting from
the collaboration. If Searle elects to commercialize a drug, Searle will fund
and perform preclinical studies and clinical trials of that drug and will be
responsible for obtaining regulatory approvals for, and marketing of, that drug.
Hybridon has agreed to perform certain research and development work exclusively
with Searle. In addition, for each drug candidate Searle must make payments to
Hybridon of up to $10,000,000 upon the achievement of development milestones.
Hybridon will also be entitled to royalties from net sales of products
commercialized as a result of the collaboration. As long as Hybridon satisfies
certain requirements relating to its manufacturing capacities and capabilities,
Hybridon will retain manufacturing rights, and Searle will be required to
purchase its requirements of bulk oligonucleotides from Hybridon on an exclusive
basis at specified prices. Upon a change in control of Hybridon, Searle would
have the right to terminate Hybridon's manufacturing rights, although the
royalty payable to Hybridon from net sales would be increased.

If Searle designates all of the additional targets or if Hybridon fails
to satisfy certain requirements relating to its manufacturing capacities and
capabilities, Searle will have the right to require Hybridon to form a joint
venture with Searle for the development and commercialization of antisense
therapeutic products in the area of inflammation/immunomodulation (other than
products relating to targets that have already been designated by Searle) to
which Searle will contribute $50,000,000 in cash and certain intellectual
property rights. Hybridon will also contribute certain intellectual property and
technology and, if the fair market value of that technology is less than
$50,000,000, Hybridon will, at its discretion, either contribute the difference
in cash or have its share of the first profits of the joint venture

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reduced by the amount of that difference. Hybridon and Searle would each own 50%
of the joint venture, although Searle's ownership interest could increase to as
much as 75% if the joint venture is established because of Hybridon's failure to
satisfy the requirements relating to its manufacturing capacities and
capabilities.

Pursuant to their collaboration, Searle also purchased 200,000 shares
of common stock in Hybridon's initial public offering.

Medtronic, Inc.

In May 1994, Hybridon and Medtronic agreed to test a device for
delivering Hybridon's antisense oligonucleotides for the treatment of
Alzheimer's disease. The agreement provides that Hybridon is responsible for the
development of, and will hold all rights to, any drug developed as a result of
this agreement and Medtronic is responsible for the development of, and will
hold all rights to, any delivery system developed as a result of this agreement.
The parties may agree to extend this collaboration to other neurodegenerative
disease targets. Hybridon is not currently conducting any activities under this
agreement.

As part of their collaboration, Medtronic purchased a total of 131,667
shares of Hybridon common stock.

HYBRIDON SPECIALTY PRODUCTS

In 1996, Hybridon formed Hybridon Specialty Products, or "HSP," to
manufacture oligonucleotide compounds both for Hybridon's internal use, for use
by its collaborators and for sale to third parties. Hybridon believes that the
current interest in genetic medicine or drugs based on genetic information will
continue, and even increase, as the potential of these technologies for the
development of new classes of drugs becomes more widely understood, and that as
a result demand for oligonucleotide compounds will increase. Hybridon's strategy
is to position HSP to take advantage of this increased demand. There can be no
assurance that this strategy will be successful or that demand will increase as
anticipated. HSP is, however, attempting to minimize this risk by manufacturing
oligonucleotides for many applications, at different stages of development. HSP
is currently manufacturing oligonucleotides for genomic, diagnostic and
therapeutic applications, and Hybridon believes HSP's customers are developing
over 20 oligonucleotide drugs, with at least eight currently in clinical
studies.

HSP manufactures oligonucleotides at its 36,000-square-foot leased
facility, which Hybridon believes is the only facility currently capable of
manufacturing oligonucleotides on a large scale. HSP first began producing
oligonucleotide compounds for sale in June 1996 and had revenues of
approximately $1.1 million in 1996, $1.9 million in 1997 and $2.8 million in
1998. HSP's principal customers in 1998 included Genta Incorporated, LaJolla
Pharmaceuticals, Inc. and MethylGene, Inc.

HSP has developed a manufacturing technology platform that combines
multiple methods to improve the production process and increase the amount of
compounds produced in a single batch, thereby permitting economies of scale. HSP
has developed two separate machines, called synthesizers, for the large-scale
synthesis of oligonucleotides. One of these machines was developed by Hybridon
alone and the other in collaboration with Pharmacia Biotech. Pharmacia has the
right to make and sell synthesizers based on the design developed in the
collaboration but must also pay Hybridon royalties. Hybridon believes that its
synthesizers are the first commercial-scale oligonucleotide synthesizers
designed for advanced oligonucleotide chemistries. In addition, HSP has
developed purification processes that use water in place of chemical solvents,
thereby decreasing the impact of the process on the environment and permitting
HSP to purify large quantities of oligonucleotides. HSP has also developed
processes and unique chemicals used in the process, which HSP believes may
further lower its production costs.

In 1996, Hybridon entered into a four-year sales and supply agreement
with the Applied Biosystems Division of Perkin-Elmer, pursuant to which
Perkin-Elmer agreed to refer potential customers to HSP, and Hybridon agreed to
purchase certain raw materials from Perkin-Elmer for the manufacture of
oligonucleotides sold to those customers. Hybridon is required to pay
Perkin-Elmer a percentage of the sales price paid by those customers. In
addition, Perkin-Elmer licensed to Hybridon its oligonucleotide synthesis
patents.

HSP is targeting three market areas for oligonucleotides: antisense
therapeutics, non-antisense therapeutics, and diagnostic/genomic DNA probes,
which are oligonucleotides designed to detect the presence of specific genes.
Within 

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each area there is a large number of potential products. HSP is currently
manufacturing oligonucleotides for customers in each of these three market
areas.

The production of oligonucleotides is similar in many respects to the
chemical synthesis used to produce conventional drugs. However, unlike many
conventional drugs, one can with the same chemical building blocks and
essentially the same manufacturing processes and equipment make different
antisense compounds for treating different diseases. As a result, the knowledge
and experience that HSP obtains manufacturing one oligonucleotide compound can
be applied to the manufacture of other oligonucleotide compounds. Furthermore,
since several different oligonucleotide compounds can be manufactured in one
facility, Hybridon anticipates that HSP will have the ability to manufacture
multiple marketed oligonucleotide-based drugs without having to build a separate
plant for each such compound.

In order to meet Hybridon's needs and satisfy outside demand, HSP may
need to increase its manufacturing capacity by adding more oligonucleotide
synthesizers. In addition, in order for Hybridon to successfully commercialize
its drugs or for HSP to achieve a satisfactory profit on sales, HSP may need to
reduce its production costs further.

Hybridon believes that it is currently manufacturing oligonucleotides
according to FDA Good Manufacturing Practices, or "GMP". The FDA has not
formally reviewed HSP's facility and procedures, and Hybridon may need to revise
those procedures in the future as production increases. Since 1996, HSP has
undergone multiple significant audits for GMP compliance conducted by
biotechnology and pharmaceutical companies. No significant deficits have been
identified. In addition, in 1997, HSP was one of two biotechnology companies
chosen to participate in the FDA's Biotechnology PAI Pilot Initiative, a pilot
program that allows FDA regulatory officials to provide advice to the selected
companies on compliance with FDA standards before they submit drug approval
filings. The FDA would have informed Hybridon of any substantial issues if any
had arisen.

MARKETING STRATEGY

Hybridon plans to market the drugs it is developing either directly,
using its own sales force, or through co-marketing, licensing, distribution or
similar arrangements with other pharmaceutical and biotechnology companies,
particularly if the products are intended to serve a large,
geographically-diverse patient population. Direct marketing of any of its
proposed drugs would require a substantial marketing staff and sales force
supported by a distribution system. Co-marketing or other arrangements with
other pharmaceutical or biotechnology companies would allow Hybridon to avoid
the significant cost involved in direct marketing, but would make Hybridon
reliant on the efforts of others. While Hybridon has developed general marketing
strategies, it has not begun to implement any of these strategies.

ACADEMIC  AND RESEARCH COLLABORATIONS

Hybridon has entered into a number of collaborative research
relationships with independent researchers and leading academic and research
institutions and U.S. government agencies, including the National Institutes of
Health, or "NIH". Such research relationships allow Hybridon to augment its
internal research capabilities and obtain access to specialized knowledge or
expertise.
In general, Hybridon's collaborative research agreements require
Hybridon to pay various amounts to support the research. Hybridon usually
provides the oligonucleotides, which the collaborator then tests. If in the
course of conducting research under its agreement with Hybridon a collaborator,
solely or jointly with Hybridon, creates any invention, Hybridon generally has
an option to negotiate an exclusive, worldwide, royalty-bearing license to the
invention. Inventions developed solely by Hybridon's scientists in connection
with a collaborative relationship generally are owned exclusively by Hybridon.
Most of these collaborative agreements are nonexclusive and can be cancelled on
short notice.

Since July 1997, as part of its restructuring, Hybridon has allowed a
number of its collaborative research agreements to expire and has terminated
certain others, but has maintained those that it believes support its current
drug discovery and development programs.

DRUG DEVELOPMENT SERVICES

Hybridon's Drug Development Department has experience in the design and
conduct of preclinical and clinical trials and has prepared and submitted
reports and other regulatory documents in connection with the three Hybridon

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advanced chemistry antisense compounds that have entered clinical studies.
Pursuant to a contract with MethylGene, Hybridon's Drug Development Department
has also used its expertise to help design and monitor the preclinical trials of
MethylGene's antisense compound, MG98, that led to MethylGene's submission of
IND applications in Canada and the U.S. MethylGene compensated Hybridon for
these services. Hybridon may perform similar services for OriGenix.

PATENTS, TRADE SECRETS, AND LICENSES

Hybridon's success will largely depend on its ability to:

o        obtain U.S. and foreign patent protection for drug candidates and
processes

o         preserve trade secrets

o        operate without infringing the proprietary rights of third parties

Hybridon's policy is to file patent applications to protect technology,
inventions and improvements that it considers important to development of its
business, and to obtain licenses to other patents that could help Hybridon
maintain or enhance its competitive position. As of January 15, 2000, Hybridon
owned or exclusively licensed in excess of 113 U.S. and foreign issued and
allowed patents, of which 79 are U.S. patents, and 59 other U.S. and 88 other
foreign patent applications. These patents and applications cover various
chemically advanced oligonucleotides, target sequences, oligonucleotide
products, methods for making and purifying oligonucleotides, analytical methods,
and methods for antisense treatment of various diseases. The patents expire on
dates ranging from 2006 to 2015.

Hybridon is the worldwide exclusive licensee under several U.S. issued
patents or allowed patent applications owned by University of Massachusetts
Medical Center, or "UMMC" (formerly the Worcester Foundation), relating to
oligonucleotides and hybrid or mixed backbone chemistries. Many of these patents
and patent applications have corresponding patents issued by, or corresponding
patent applications on file in, other major industrial countries. One of the
issued U.S. patents and one of the issued European patents cover antisense
oligonucleotides as new compositions of matter for stopping the replication of
HIV. Coverage of the other issued U.S. patents includes composition and use of
oligonucleotides based on advanced chemistries, methods of oligonucleotide
production, composition of certain modified oligonucleotides that are useful for
diagnostic tests or assays, and methods of purifying oligonucleotides. The UMMC
patents licensed to Hybridon expire at various dates starting in 2006.

Hybridon is the exclusive licensee under various other U.S. and foreign
patents and patent applications, including two U.S. patent applications owned by
McGill University relating to oligonucleotides and DNA methyltransferase.
Hybridon and Massachusetts General Hospital jointly own one issued U.S. patent
applicable to Alzheimer's disease. Hybridon holds an exclusive license to
Massachusetts General Hospital's interests under this patent.

Hybridon is a nonexclusive licensee of certain patents held by the
National Institutes of Health, or "NIH," relating to oligonucleotide
phosphorothioates and is a nonexclusive licensee of an NIH patent covering the
phosphorothiolation of oligonucleotides. The field of each of these licenses
extends to a wide variety of genetic targets. Hybridon is also a nonexclusive
licensee of certain patents exclusively licensed to Genzyme covering certain
technology relating to MDM2.

The U.S. Patent and Trademark Office, or "PTO," has informed Hybridon
that certain patent applications exclusively licensed by Hybridon from UMMC will
be submitted to the Board of Patent Appeals and Interferences of the PTO to
determine whether an interference should be declared with issued U.S. patents
held by the NIH relating to oligonucleotide phosphorothioates. An interference
proceeding is a proceeding to determine who was the first to invent, and thus
who is entitled to a patent for, a claimed invention. McDonnell Boehnen Hulbert
& Berghoff, a U.S. patent counsel for Hybridon, is of the opinion that the UMMC
patent application has a prima-facie case for priority against the NIH for an
invention that includes phosphorothioate-modified oligonucleotides. There can be
no assurance, however, that the PTO will declare an interference, or if it does,
what the outcome will be. If Hybridon were to lose the interference, its
nonexclusive license from the NIH of the NIH phosphorothioate patents would not
be affected. If Hybridon were to win the interference, others making, using or
selling certain phosphothioate-modified oligonucleotides would be required to
obtain a license from Hybridon.

The PTO also declared a four-way interference involving two UMMC U.S.
patents, for which Hybridon is the exclusive licensee, relating to a particular
type of modified oligonucleotides. The other parties to this interference were

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Integrated DNA Technologies, or "IDT," Isis Pharmaceuticals, Inc. and Gilead
Sciences, Inc. This interference was settled in early 1999. In connection with
the settlement, Hybridon has obtained a nonexclusive license to certain patents
and patent applications owned by IDT that broadly claim chemical modifications
to oligonucleotides. Hybridon has also granted a nonexclusive license to IDT to
make, use, and sell limited quantities of oligonucleotides incorporating certain
of Hybridon's advanced chemistries.

Under its licenses, Hybridon is obligated to pay royalties on its net
sales of products or processes covered by the licensed technology and, in some
cases, to pay a percentage of sublicense income that it receives. These licenses
impose various commercialization, sublicensing, insurance and other obligations
on Hybridon. If Hybridon fails to comply with these requirements, the license
could be terminated.

Legal standards relating to the validity of patents covering
pharmaceutical and biotechnological inventions and the scope of claims made
under such patents are still developing. As a result, Hybridon's ability to
obtain and enforce patents that protect its drugs is uncertain and involves
complex legal and factual questions.

That Hybridon owns or licenses pending or future patent applications
does not mean that patents based on those applications will ultimately be
issued. First, to obtain a patent on an invention, one must be the first to
invent it or the first to file a patent application for it. Patent applications
in the U.S. are maintained in secrecy until patents issue, and publication of
any given discovery in the scientific or patent literature tends to lag behind
actual date of that discovery by several months. Consequently, Hybridon cannot
be certain that the inventors of subject matter covered by patents and patent
applications that it owns or licenses were the first to invent, or the first to
file patent applications for, those inventions.

Others, including Hybridon's competitors, also hold issued patents and
patent applications relating to antisense technology or particular genetic
targets, including an issued patent in Europe covering the gene MDM2. Holders of
any of these patents or patent applications may be able to require Hybridon to
change or cease making or using certain products or processes, or obtain an
exclusive or nonexclusive license in return for licensing fees, which may be
substantial. Hybridon may not be able to obtain any such licenses at a
reasonable cost. Furthermore, such licenses may be made available to competitors
of Hybridon on an exclusive or nonexclusive basis. Failure to obtain such
licenses could have a material adverse effect on Hybridon. Previously, a
competitor was granted another European patent relating to certain types of
stabilized synthetic oligonucleotides for use as therapeutic agents for
selectively blocking the translation of a messenger RNA into a targeted protein
by binding with a portion of the messenger RNA to which the stabilized synthetic
oligonucleotide is substantially complementary. This European patent was revoked
in its entirety in an opposition proceeding before the European Patent Office in
September 1995. The holder of this patent appealed this decision. This appeal
was dismissed on February 18, 1999.

Hybridon requires its employees, consultants, outside scientific
collaborators, sponsored researchers and other advisors to execute
confidentiality agreements. These agreements provide that all confidential
information developed or made known by Hybridon to the individual is to be kept
confidential, subject to specific exceptions. In the case of employees, the
agreements provide that all inventions conceived by the individual are the
exclusive property of Hybridon. These agreements may not, however, provide
meaningful protection for Hybridon's trade secrets or adequate remedies in the
event of breach.

Consistent with pharmaceutical industry and academic standards,
Hybridon's agreements with academic and research institutions and U.S.
government agencies may provide that the results of a given collaboration, or
any developments that derive from the collaboration, will be freely published,
that information or materials supplied by Hybridon will not be treated as
confidential, and that Hybridon must negotiate a license to developments and
results in order to commercialize products incorporating them. There can be no
assurance that Hybridon will be able successfully to obtain any such license at
a reasonable cost or that such developments and results will not be made
available to competitors of Hybridon on an exclusive or nonexclusive basis. See
"Business--Academic and Research Collaborations."

GOVERNMENT REGULATION

Hybridon's research, clinical development and production activities are
regulated for safety, effectiveness and quality by numerous governmental
authorities in the U.S. and other countries. Hybridon believes that it is in
material compliance with all applicable federal, state and foreign legal and
regulatory requirements.

-17-

FDA Approvals. In addition to product approvals by the FDA, as
described above, the FDA may require that it inspect Hybridon's manufacturing
facilities for compliance with GMP and other applicable rules and regulations
before it will permit a product manufactured by Hybridon to be marketed in the
U.S. Any material change by Hybridon in its manufacturing process or equipment,
including relocation of the manufacturing facility, would necessitate additional
FDA review and approval.

Other Regulation. In addition to regulations enforced by the FDA,
Hybridon also is subject to regulation under the Occupational Safety and Health
Act and other present and potential future federal, state or local regulations.
Furthermore, because Hybridon uses hazardous materials, chemicals, viruses, and
various radioactive compounds, it must comply with U.S. Department of
Transportation and Environmental Protection Agency regulations and other
federal, state, and foreign laws and regulations regarding hazardous waste
disposal, air emissions, and waste-water discharge. Although Hybridon believes
that it complies with these laws and regulations, it cannot completely eliminate
the risk of accidental contamination or injury from these materials.

COMPETITION

There are a number of companies, both privately and publicly held, that
are conducting research and development activities on technologies and products
aimed at therapeutic regulation of gene expression, including antisense drugs.
One competitor of Hybridon has recently received FDA approval to market an
antisense therapeutic product for the treatment of CMV retinitis. Hybridon
believes that the interest in these technologies and products will increase. It
is possible that Hybridon's competitors will succeed in developing products that
are more effective than Hybridon's. Furthermore, Hybridon's proposed drugs will
be competing with other kinds of drugs. Given the fundamental differences
between antisense technology and other drug technologies, antisense drugs may be
less effective at treating some diseases than other kinds of drugs.

Biotechnology and related pharmaceutical technologies have undergone
and continue to be subject to rapid and significant change. Hybridon expects
that the technologies associated with biotechnology research and development
will continue to develop rapidly. Hybridon's future will depend in large part on
its ability to compete with these technologies

Hybridon has many competitors, including major pharmaceutical and
chemical companies, biotechnology firms, and universities and other research
institutions. Many of these competitors have substantially greater financial,
technical, and human resources than Hybridon, and many have significantly
greater experience than Hybridon in undertaking preclinical studies and clinical
trials of new pharmaceutical products and obtaining FDA and other regulatory
approvals. Accordingly, Hybridon's competitors may succeed in obtaining
regulatory approvals for products more rapidly than Hybridon. Furthermore, if
Hybridon receives approval to commence commercial sales of products, it will
also be competing with respect to manufacturing efficiency and marketing
capabilities, areas in which it has limited experience.

HSP also faces competition, as Hybridon's customers may begin to
produce oligonucelotides internally or may find other sources. Hybridon may be
forced to reduce the cost of its products to meet the competition.

EMPLOYEES

As of January 24, 2000, Hybridon employed 45 individuals full-time, of
whom 15 held advanced degrees. Eight of these employees are engaged in research
and development activities and ten are employed in finance, corporate
development, and legal and general administrative activities. In addition, 27 of
these employees are employees of HSP, of whom 5 are employed in quality control.
Many of Hybridon's management and professional employees have had prior
experience with pharmaceutical, biotechnology, or medical products companies.
None of Hybridon's employees is covered by a collective bargaining agreement,
and management considers relations with its employees to be good.

PROPERTIES

Hybridon leases its 36,000 square foot facility in Milford,
Massachusetts under a lease that expires in 2004. Hybridon has an option to
extend this lease for two additional five-year terms.

-18-

In addition, Hybridon leases approximately 26,000 square feet of
supplemental laboratory space in Cambridge, Massachusetts under a lease that
expires April 30, 2007. The annual rent for this space is approximately $23 per
square foot. Hybridon is currently subleasing approximately 20,000 square feet
of this to a third party under a sublease that expires September 30, 2000.

LEGAL PROCEEDINGS

Hybridon is not a party to any litigation that it believes could damage
Hybridon or its business.

MARKET FOR REGISTRANT'S COMMON EQUITY AND RELATED STOCKHOLDER MATTERS

From January 24, 1996 until December 2, 1997, Hybridon's common stock
was traded on the Nasdaq National Market under the symbol "HYBN." Prior to
January 24, 1996, there was no established public trading market for Hybridon's
common stock.

On December 2, 1997, Hybridon's common stock was removed from the
Nasdaq National Market and began being quoted on the NASD OTC Bulletin Board.
Quotes on the NASD OTC Bulletin Board may reflect inter-dealer prices, without
retail markups, markdowns or commissions and do not necessarily represent actual
transactions.

On December 10, 1997 Hybridon effected a one-for-five reverse stock
split of its common stock. As a result of the reverse stock split, each five
shares of common stock was automatically converted into one share of common
stock, with cash payments for any fractional shares.

The following table sets forth for the periods indicated the high and
low sales prices per share of the common stock during each of the quarters set
forth below as reported on the Nasdaq National Market and the NASD OTC Bulletin
Board since January 1, 1998:

HIGH            LOW
----            ---

1998

First Quarter..........................................     $3.359      $1.000
Second Quarter.........................................      2.75        1.609
Third Quarter..........................................      2.516       1.125
Fourth Quarter.........................................      3.25        1.125

1999

First Quarter..........................................    $1.875      $1.000
Second Quarter.........................................      1.50       0.250
Third Quarter..........................................      1.50       0.350
Fourth Quarter.........................................      1.75       0.406

The reported closing bid price of the common stock on the NASD OTC
Bulletin Board on February 2, 2000 was $1.75 per share.

DIVIDEND POLICY

The convertible preferred stock pays dividends at 6.5% per year,
payable semi-annually in arrears. These dividends may be paid either in cash or
in additional shares of convertible preferred stock, at the discretion of
Hybridon.

-19-

Hybridon has never declared or paid cash dividends on its capital
stock, and Hybridon does not expect to pay any dividends on its common stock or
any cash dividends on the convertible preferred stock in the foreseeable future.
The indenture under which Hybridon issued 9% convertible subordinated notes on
April 2, 1997, limits Hybridon's ability to pay dividends or make other
distributions on its common stock or to pay cash dividends on the convertible
preferred stock. As of January 31, 2000, $1.3 million in total principal amount
of the 9% notes remained outstanding.

In addition, Hybridon is currently prohibited from paying cash
dividends under the loan held by the Lender. See "Management's Discussion and
Analysis of Financial Condition and Results of Operation--1998 Financing
Activities--Credit Facility."

USE OF PROCEEDS

Hybridon will not receive any proceeds from the sale of the securities
by selling stockholders other than proceeds upon exercise of certain Hybridon
warrants. Those proceeds will be added to Hybridon's general working capital.

-20-

SELECTED FINANCIAL DATA

The selected balance sheet data set forth below, as of December 31,
1997 and 1998, and the statements of operations data for each of the three years
in the period December 31, 1998, come from Hybridon's consolidated financial
statements which have been audited by Arthur Andersen LLP, independent public
accountants, and which are included elsewhere in this prospectus. The selected
financial data as of December 31, 1994, 1995 and 1996 and for the years ended
December 31, 1994 and 1995 come from Hybridon's consolidated financial
statements not included in this prospectus, all of which have been audited by
Arthur Andersen LLP, independent public accountants. The selected financial data
as of September 30, 1999 and for the nine months ended September 30, 1998 and
1999 come from Hybridon's unaudited consolidated financial statements which are
included elsewhere in this prospectus and which include, in the opinion of
Hybridon, all normal recurring adjustments that are necessary for a fair
presentation of its financial position and the results of its operations for
those periods. Operating results for the nine months ended September 30, 1999
may not be indicative of the results that may be expected for the fiscal year
ending December 31, 1999. The selected financial data should be read along with,
and are qualified by reference to, "Management's Discussion and Analysis of
Financial Condition and Results of Operations," Hybridon's consolidated
financial statements and notes thereto and the Report of Independence Public
Accountants included elsewhere in this prospectus.

Years Ended December 31, 
-------------------------------------------------------
1994       1995         1996         1997      1998 
----       ----         ----         ----      -----
(In Thousands, except per share data)

Statement of Operations Data:
Revenues:
Product and service revenue .............   $     --    $     --    $  1,080    $  1,877    $  3,254
Research and development ................      1,032       1,186       1,419         945       1,100
Royalty income ..........................         --          --          62          48          --
Interest income .........................        135         219       1,447       1,079         148
Total revenues ..................      1,167       1,405       4,008       3,949       4,502

Operating Expenses :
Research and development ................     20,024      29,685      39,390      46,828      20,977
General and administrative ..............      6,678       6,094      11,347      11,026       6,573
Interest ...............................         69         173         124       4,536       2,932
Restructuring ...........................         --          --          --      11,020          --
--------    --------    --------    --------    --------

Total operating expenses ...............     26,771      35,952      50,861      73,410      30,482
Loss from operations .........................    (25,604)    (34,547)    (46,853)    (69,461)    (25,980)
Extraordinary item:
Gain on conversion of 9% convertible ....         --          --          --          --       8,877
--------    --------    --------    --------    --------
subordinated notes payable
Net loss .....................................    (25,604)    (34,547)    (46,853)    (69,461)    (17,103)
Accretion of preferred stock dividend ........         --          --          --          --       2,689
--------    --------    --------    --------    --------
Net loss to common stockholders ..............   $(25,604)   $(34,547)   $(46,853)   $(69,461)   $(19,792)

Basic and diluted net loss per  common share     $ (70.77)   $ (94.70)   $ (10.24)   $ (13.76)   $  (2.19)
from:
Operations
Extraordinary gain .....................         --          --          --          --        0.75
--------    --------    --------    --------    --------
Net loss per share ......................     (70.77)     (94.70)     (10.24)     (13.76)      (1.44)
Accretion of preferred stock dividends ..         --          --          --          --       (0.23)
--------    --------    --------    --------    ========
Net loss per share applicable to common
stockholders ...........................   $ (70.77)   $ (94.70)   $ (10.24)   $ (13.76)   $  (1.67)
Shares Used in Computing Basic and
Diluted Net Loss per Common Share(1) ....        362         365       4,576       5,050      11,859

-21-

Balance Sheet Data: 

December  31,
-------------------------------------------------------------
1994         1995        1996        1997          1998
----         ----        ----        ----          ----

Cash, cash equivalents and short-term      $   3,396    $   5,284    $  16,419    $   2,202    $   5,607
investments(2)
Working capital (deficit) ..............      (1,713)         210        8,888      (24,100)      (5,614)
Total assets .........................      11,989       19,618       41,537       35,072       16,536
Long-term debt and capital lease
obligations, net of current portion       1,522        1,145        9,032        3,282          473
9% convertible subordinated
notes payable ..........................          --           --           --       50,000        1,306
Accumulated deficit ....................     (67,794)    (102,341)    (149,194)    (218,655)    (238,448)
Total stockholders' equity (deficit) ...       4,774       12,447       22,855      (46,048)       2,249

(1)  Computed on the basis described in Notes 2(k) of Notes to consolidated
financial statements appearing elsewhere in this prospectus.

(2)  Short-term investments consisted of U.S. government securities with
maturities greater than three months but less than one year from the
purchase date.

-22-

Nine Months
Ended September 30,
----------------------
1998          1999
(Unaudited)
Statement of Operations Data:
Revenues:
Product and service revenue ................   $  2,353    $  4,644
Research and development ...................        950         450
Royalty income .............................         --         107
Interest income ............................        106          82
Total revenues ....................      3,409       5,283

Operating Expenses:
Research and development ...................     17,181      10,106
General and administrative .................      5,818       2,947
Interest ...................................      2,880         562
Restructuring ..............................         --          --

Total operating expenses ...................     25,879      13,615
Loss from operations ............................    (22,470)     (8,332)
Extraordinary item:
Gain on conversion of 9% convertible .......      8,877          --
subordinated notes payable
Net loss ........................................    (13,593)     (8,332)
Accretion of preferred stock dividend ...........      1,647       3,194
Net loss to common stockholders .................   $(15,240)   $(11,526)

Basic and diluted net loss per common share from:   $  (2.11)   $  (0.54)
Operations
Extraordinary gain .........................       0.83          --
Net loss per share .........................      (1.28)      (0.54)
Accretion of preferred stock dividends ....      (0.15)      (0.20)
Net loss per share applicable to common
stockholders ..........................   $  (1.43)   $  (0.74)
Shares Used in Computing Basic and ..............     10,648      15,654
Diluted Net Loss per Common Share(1)

-23-

Balance Sheet Data:                                            September 30,
1999
---------------
(Unaudited)
Cash, cash equivalents and short-term ......................        $     500
investments(2)
Working capital (deficit) ..................................          (10,540)
Total assets ...............................................            9,193
Long-term debt and capital lease
obligations, net of current portion .....................              414
9% convertible subordinated
notes payable ............................................            1,306
Accumulated deficit ........................................         (249,974)
Total stockholders' equity (deficit) .......................           (4,507)

(1)  Computed on the basis described in Notes 2(K) of Notes to consolidated
financial statements appearing elsewhere in this prospectus.

(2)  Short-term investments consisted of U.S. government securities with
maturities greater than three months but less than one year from the
purchase date.

-24-

MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION
AND RESULTS OF OPERATIONS

RECENT FINANCING ACTIVITIES

The discussion in the sections below relates to Hybridon's financial
condition and results of operations through September 30, 1999, which is the
period covered by the financial statements included elsewhere in this
prospectus. This section summarizes developments since September 30, 1999.

Hybridon sold an aggregate of $1,500,000 principal amount of promissory
notes to E. Andrews Grinstead, III, Hybridon's Chief Executive Officer, at face
value during September and November of 1999. These notes accrue interest at 12%
per annum (15% if Hybridon elects to pay this interest in shares of common stock
rather than cash). It was intended that upon the closing of any third-party debt
financing on or before March 1, 2000, these notes would be converted into the
debt sold in that financing, and in December 1999 they were converted into 8%
notes of Hybridon due 2002. Hybridon also sold an aggregate of approximately
$525,000 of debt to purchasers in a private placement transaction in October and
November 1999; as of December 13, 1999, this debt automatically converted into
8% notes.

On December 13, 1999, Hybridon sold an aggregate of an additional $5.1
million principal amount of 8% notes to purchasers in a private placement
transaction. Including the 8% notes issued upon conversion of the debt issued to
Mr. Grinstead and other purchasers, there is $7.1 million principal amount of 8%
notes outstanding. These notes earn interest semi-annually at 8% per annum,
mature on November 30, 2002 and are convertible into Hybridon's common stock at
an initial conversion price of $.60 per share.

In connection with the offering of these notes, Forum and the entities
advised by Pecks entered into a Subordination and Intercreditor Agreement with
Hybridon and the representative of the purchasers of the notes whereby, among
other things, they agreed to subordinate their loan to the notes, subject to
certain conditions. Also in connection with this offering, Hybridon agreed to
issue warrants to purchase an aggregate of 2.75 million shares of Hybridon's
common stock to designees of Pecks and Forum. These warrants are exercisable
from December 31, 2000 until December 31, 2002 at $.60 per share.

The notes permit the noteholders' representative to declare an event of
default, among other things, if Hybridon fails to maintain, as of the last day
of any calendar month, consolidated cash on hand (and cash equivalents and
marketable securities) of at least $1.5 million. As of January 31, 2000,
Hybridon met this requirement. If an event of default under the notes were
declared and not cured in the requisite time period, then the respective
representatives of the notesholders, Forum and the entities advised by Pecks
could declare their debt securities immediately due and payable, in which case
Hybridon may be required to sell substantial assets to raise funds for this
repayment and, if the proceeds of those sales together with any other funds
available are insufficient, Hybridon could be forced to declare bankruptcy.

GENERAL

Hybridon's existing cash resources are expected to be sufficient to
fund operations up to June 2000. Hybridon's ability to continue operations
beyond that time will depend on its success in obtaining new funding, either
through additional financing or new partnerships or collaborations with third
paries, particularly if its existing collaboration with Searle is terminated.
See "Business- Corporate Collaborations-G.D. Searle & Co." If Hybridon is unable
to obtain substantial additional new funding by June 2000, Hybridon may have to
terminate operations or seek relief under applicable bankruptcy laws.

Hybridon is involved in the discovery and development of genetic
medicines based on antisense technology. Hybridon began operations in February
1990 and since that time has been involved primarily in research and development
efforts, developing its manufacturing capabilities, and raising capital. In
order to commercialize its therapeutic products, Hybridon will need to address a
number of technological challenges and comply with comprehensive regulatory
requirements. All revenues received by Hybridon to date have been come from
collaborative agreements, interest on invested funds and revenues from the
custom contract manufacturing of synthetic DNA and reagent products by Hybridon
Specialty Products.

Hybridon has had total losses of approximately $250 million through
September 30, 1999. Hybridon adopted a restructuring plan in the second half of
1997 that has significantly reduced its operating expenses. However, Hybridon
expects that its research and development expenses will be significant in 1999
and future years as it pursues its core drug development programs and expects to
continue to have operating losses and significant capital needs beyond its
internally generated funds. As of January 24, 2000, Hybridon has 45 full-time
employees.

-25-

RESTRUCTURING  PLAN

During the second half of 1997, Hybridon adopted a restructuring plan
to reduce spending in order to save money. As part of this plan, in addition to
stopping the development of GEM(R) 91, Hybridon limited or suspended programs
unrelated to its main advanced chemistry antisense drug development programs. In
addition, in 1997, Hybridon ended the employment of a substantial number of
employees at its Cambridge and Milford, Massachusetts and Paris, France
facilities and substantially limited operations at its Paris, France office. In
December 1998, Hybridon began the final process of ending all operations in
Europe.

In 1997 Hybridon subleased a portion of each of its facilities in
Cambridge, Massachusetts (including a substantial portion of its former
headquarters). In June 1998, Hybridon moved its headquarters from Cambridge,
Massachusetts to its facility in Milford, Massachusetts and then sold its
interest in Charles River Building Limited Partnership, or the "Cambridge
Landlord," which owned the former Cambridge headquarters. As a result, Hybridon
received $6,163,000 in cash, which included the return of a portion of its
security deposit for its Cambridge headquarters and the reclassification on
Hybridon's balance sheet of $660,000 from restricted cash to cash and cash
equivalents. The Cambridge facility was leased in September 1998 to a third
party, subject to a sublease of a portion of the premises. As a result of these
actions, Hybridon was relieved of its substantial lease obligations for the
Cambridge facility, subject to a continuing liability for any defaults which may
arise under the sublease.

RESULTS OF  OPERATIONS

Nine months ended September 30, 1999 and 1998

Hybridon had total revenues of $5.3 million and $3.4 million for the
nine months ended September 30, 1999 and 1998, respectively. Revenues from
products and services were $4.6 million and $2.4 million for the nine months
ended September 30, 1999 and 1998, respectively. The increase was primarily the
result of increased sales to Hybridon Specialty Products customers and receipt
of service revenues from MethylGene, Inc., an entity in which Hybridon has an
approximately 30% equity interest , and OriGenix Technologies, Inc., an entity
in which Hybridon has an approximately 49% equity interest. The revenues
received from MethylGene decreased from $1.6 million to $0.9 million and
increased for OriGenix from zero to $76,000 for the nine months ended September
30, 1998 and 1999, respectively.

Revenues from research and development collaborations were $0.5 million
and $0.9 million for the nine months ended September 30, 1999 and 1998,
respectively. This decrease was primarily due to a reduction in revenues
recorded under a License Agreement with MethylGene, Inc.

Hybridon's research and development expenses were $10.1 million and
$17.2 million for the nine months ended September 30, 1999 and 1998,
respectively. The decrease reflects Hybridon's reduction of its operating
expenses in 1997 and 1998 pursuant to the restructuring that began in 1997 and
was completed in 1998 and the lower levels of cash available for expenditures in
1999. The restructuring included ending operations at Hybridon's facilities in
Europe, and also resulted in significant reductions in employees and
employee-related expenses, clinical and outside testing, consulting, materials
and lab expenses.

In addition, the facilities expense included in research and
development expenses decreased significantly in 1999 as a result of moving
Hybridon's corporate offices and lab space in July 1998 from Cambridge to
Milford, Massachusetts and the sublease of its remaining unused Cambridge
facilities .

Hybridon's general and administrative expenses were $2.9 million and
$5.8 million for the nine months ended September 30, 1999 and 1998,
respectively. The decrease reflects Hybridon's reduction of its operating
expenses in 1997 and 1998 pursuant to the restructuring which began in 1997 and
completed in 1998 and which resulted in significant reduction in employees and
employee-related expenses and consulting expenses . General and administrative
expenses related to business development, public relations and legal and
accounting expenses also decreased in 1999.

In addition, the facilities expense included in general and
administrative expenses also decreased significantly in 1999 as a result of
moving Hybridon's corporate offices to Milford, Massachusetts in 1998.

-26-

Hybridon's patent expenses remained at approximately the same level in
1999 as 1998.

Hybridon's interest expense was $0.6 million and $2.9 million for the
nine months ended September 30, 1999 and 1998, respectively. The decrease is
attributable to the exchange of approximately $48.7 million of the 9%
convertible subordinated notes issued in the second quarter of 1997 for Series A
preferred stock on May 5, 1998. In addition, the outstanding balance of
borrowings to finance the purchase of property and equipment was reduced in May
1998, resulting in a subsequent reduction in interest expense.

As a result of the above factors, Hybridon incurred net losses from
operations of $8.3 million and $22.5 million for the nine months ended September
30, 1999 and 1998, respectively.

Hybridon had extraordinary income of $8.9 million for the nine months
ended September 30, 1998 resulting from the conversion of $48.7 million
principal amount of its 9% notes to Series A preferred stock in the second
quarter of 1998. In accordance with Statement of Financial Accounting Standards
No. 15, Accounting by Debtors and Creditors for Troubled Debt Restructurings,
Hybridon recorded an extraordinary gain of approximately $8.9 million related to
the exchange. The extraordinary gain represents the difference between the
carrying value of the 9% notes offered for exchange and the fair value of the
Series A preferred stock issued upon the exchange, as determined by the per
share sales price of such stock sold in May 1998 in the private offering
described below. As a result of this transaction, Hybridon reduced its net loss
to $13.6 million for the nine months ended September 30, 1998.

Hybridon had preferred stock dividends of $3.2 million and $1.6 million
for the nine months ended September 30, 1999 and 1998, respectively, reflecting
the accrued portion of dividends payable to the holders of Series A preferred
convertible stock, resulting in a net loss to common stockholders of $11.5
million and $15.2 million for the nine months ended September 30, 1999 and 1998,
respectively.

Years ended December 31, 1996, 1997 and 1998

Hybridon had total revenues of $4.0 million in 1996, $3.9 million in
1997, and $4.5 million in 1998. During 1996, 1997 and 1998, Hybridon received
revenues from research and development collaborations of $1.4 million, $0.9
million and $1.1 million, respectively. Research and development collaboration
revenues decreased in 1997 from 1996 because of the cancellation by Roche of its
collaboration with Hybridon and the resulting elimination of research funding by
Roche. Research and development collaboration revenues increased in 1998 from
1997, primarily due to Hybridon receiving certain payments under its license
agreement with MethylGene, Inc.

Product and service revenues were $1.1 million in 1996, $1.9 million in
1997 and $3.3 million in 1998. The increase in revenues in 1997 over those in
1996 resulted from a full year of operations for Hybridon Specialty Products,
which commenced operations in the third quarter of 1996. As of December 31,
1998, Hybridon Specialty Products had a backlog of $0.9 million. The increase in
revenues in 1998 was primarily the result of an expansion by Hybridon Specialty
Products in the customer base and increased sales to certain existing customers,
and was also due in part to Hybridon receiving $0.4 million in service revenue
from MethylGene.

Revenues from interest income were $1.4 million in 1996, $1.1 million
in 1997 and $0.1 million in 1998. The decrease in interest income in 1997 from
1996, and in 1998 from 1997, was the result of lower cash balances available for
investment each year.

During 1996, 1997 and 1998, Hybridon expended $39.4 million, $46.8
million and $21.0 million, respectively, on research and development activities.

The increases in research and development expenses in 1997 from 1996
reflected increasing expenses related primarily to ongoing clinical trials of
Hybridon's product candidates, including (a) clinical trials of two different
formulations of GEM(R) 132, which were first initiated during the third quarter
of 1996 and the first quarter of 1997, (b) clinical trials of GEM(R) 92, which
were initiated in the third quarter of 1997 and (c) clinical trials of GEM(R)
91, which were initiated in France in October 1993 and in the U.S. in May 1994,
and were terminated in July 1997. Clinical expenses related to GEM(R) 91
decreased significantly during the second half of 1997 after Hybridon terminated
development of this compound. Research and development expenses also increased
in 1997 over 1996 due to significant

-27-

increases in preclinical expenses incurred to meet the filing requirements to
begin clinical trials of Hybridon's product candidates in the U.S.

The decrease in research and development expenses in 1998 reflects
Hybridon's restructuring that began during the second half of 1997. The
restructuring included ending operations at Hybridon's facilities in Europe,
stopping the clinical development of GEM(R) 91 and limiting or suspending
selected programs unrelated to Hybridon's main advanced chemistry antisense drug
development program. The restructuring resulted in significant reductions in
employee-related expenses, clinical and outside testing, consulting, materials
and lab expenses.

The facilities expense related to the research and development area
increased significantly in 1997 as a result of moving the corporate offices to
Cambridge, Massachusetts and decreased significantly in 1998 as a result of
moving in July 1998 from Cambridge to Milford, Massachusetts. Hybridon's
facility costs in 1998 related to research and development were also reduced by
the income received from subleasing its Cambridge facilities.

Research and development salaries and related costs remained at
approximately the same level in 1997 as 1996 because of the costs involved in
releasing employees in 1997. Research and development salaries and related costs
decreased in 1998 from 1997 due to the substantial reduction in the number of
employees involved in research and development in 1998.

Patent expenses also remained at approximately the same level in 1998
as 1997 and 1996, as Hybridon continued to limit the scope of patent protection
that it sought as part of its effort to conserve its cash resources, while
prosecuting and maintaining key patents and patent applications.

Hybridon incurred general and administrative expenses of $11.3 million
in 1996, $11.0 million in 1997 and $6.6 million in 1998. The decrease in general
and administrative expenses in 1998 resulted primarily from Hybridon's
restructuring program which began during the second half of 1997 and its effect
on employee-related and consulting expenses and net facilities costs.
The facilities expense related to the general and administrative area
increased significantly in 1997 over 1996 as a result of moving the corporate
offices to Cambridge, Massachusetts. However, as a result of adopting the
restructuring plan in the second half of 1997, such increase was offset by
decreases in general and administrative salaries and related costs and in
consulting expenses in the second half of 1997, which carried over into 1998.
Hybridon's facilities expense related to the general and administrative area
decreased significantly in 1998 as a result of its moving to Milford,
Massachusetts. Facility costs in 1998 were also reduced by the income received
from subleasing Cambridge facilities. General and administrative expenses
related to business development, public relations and legal expenses decreased
in 1998 from 1997, but remained at approximately the same level in 1997 as 1996.

Interest expense was $0.1 million in 1996, $4.5 million in 1997 and
$2.9 million in 1998. The decrease in interest expense in 1998 is mainly due to
the exchange of approximately $48.7 million of its 9% notes for Series A
preferred stock on May 5, 1998. In addition, the outstanding balance of debt to
finance the purchase of property and equipment was reduced in May 1998,
resulting in a reduction in interest expense. The increase in interest expense
in 1997 from 1996 reflected an increase in Hybridon's debt outstanding
associated with the issuance of its 9% notes and interest incurred on borrowings
to finance the purchase of property and equipment.

As a part of its restructuring plan, Hybridon recorded an $11.0 million
restructuring charge in 1997 to provide for (i) the termination costs of certain
research programs and other contracts, (ii) the loss of certain leased
facilities, net of sublease income and other contracts, (iii) severance,
benefits and related costs for 95 terminated employees and (iv) the write down
of assets to net realizable value.

As a result of the above factors, Hybridon incurred net losses before
extraordinary items of $46.9 million in 1996, $69.5 million in 1997 and $26.0
million in 1998. Hybridon had extraordinary income of $8.9 million in 1998
resulting from the exchange of 9% notes for Series A preferred stock in the
second quarter of 1998. As a result of this transaction, Hybridon reduced its
net loss before preferred stock dividends to $17.1 million in 1998. Hybridon had
an accretion of preferred stock dividends of $2.7 million at December 31, 1998
to reflect the 1998 portion of dividends payable to the holders of Series A
preferred stock, resulting in a net loss to common stockholders of $19.8 million
for 1998.

-28-

LIQUIDITY AND CAPITAL RESOURCES

The following discussion pertains to Hybridon's financial position as of
September 30, 1999.  For a discussion of Hybridon's current financial position,
see "Recent Financing Activities," above.  

During the nine months ended September 30, 1999, Hybridon used
approximately $6.0 million to fund operating activities. The primary use of cash
for operating activities was to fund a portion of Hybridon's operating loss of
$8.3 million.

Hybridon had cash and cash equivalents of $0.5 million at September 30,
1999. However, since that date, Hybridon has spent a portion of such cash
resources and continues to have substantial obligations to lenders, real estate
landlords, trade creditors and others. On November 12, 1999, Hybridon's
obligations included a $1.0 million loan described below with E. Andrews
Grinstead, III, Hybridon's Chief Executive Officer, $1.3 million principal
amount of 9% notes, a $6.0 million loan with Forum Capital Markets, LLC and
others (collectively, the "Lenders"), a $0.5 million loan as described below,
approximately $0.5 million in 8% convertible notes as described below, and
approximately $2.5 million of accounts payable. Because of Hybridon's financial
condition, many trade creditors are only willing to provide Hybridon with
products and services on a cash on delivery basis. The note to the Lenders
contains certain financial covenants that require Hybridon to maintain minimum
tangible net worth and minimum liquidity. Hybridon is not in compliance with
those covenants. However, Forum Capital Markets has granted Hybridon a waiver of
compliance with the minimum tangible net worth requirement and the minimum
liquidity requirement at September 30, 1999 and has agreed not to require that
Hybridon comply with those requirements for any periods commencing October 1,
1999 through November 30, 1999. A representative of the other Lenders has
indicated informally to Hybridon that the other Lenders intend to do likewise,
but they have not yet entered into a written agreement to that effect.

On September 1, 1999 and September 27, 1999, Hybridon entered into two
six-month, $500,000 promissory notes payable and a loan agreement with E.
Andrews Grinstead, III. The loan is payable with interest, at the option of the
lender, at the rate of either (a) 12% per annum, payable in cash or (b) 15% per
annum, payable in Hybridon's common stock at the rate of $0.50 per share.
Interest is due and payable monthly in arrears on the first business day of each
month commencing on October 1, 1999 until March 1, 2000. The loan agreement
provides that it is the intent of the parties that upon the closing of any third
party debt financing on or before March 1, 2000, this loan will be converted
into a portion of the credit facility made pursuant to such debt financing. If
for any reason the third party debt financing does not close on or before March
1, 2000, the lender will have the option (a) to convert the entire loan to a
five-year term loan bearing interest at 8% per annum, with the right to receive
warrants to purchase in the aggregate 2,100,000 shares of Hybridon common stock
at per-share exercise prices of $1.50 (for three-year warrants) and $1.25 (for
four-year warrants), subject to downward adjustment, at the one-year anniversary
of the warrant issuance date, to the per-share market price of Hybridon's common
stock, in the case of the warrants having a $1.50 exercise price, and to 83.3%
of the per-share market price of Hybridon's common stock, in the case of the
warrants with a $1.25 exercise price, if the market price does not exceed $1.50,
(b) to convert the entire loan to a demand loan bearing interest at the lender's
option at either (i) 12% per annum, payable in cash or (ii) 15% per annum,
payable in Hybridon's common stock at the rate of $0.50 per share, or (c) to
declare the entire principal and interest immediately due and payable. The loan
may be prepaid without premium or penalty at any time. The loan is secured by
substantially all the assets of Hybridon.

During October and November 1999, Hybridon raised approximately
$500,000 under a loan agreement with various parties. The loan will be
converted, at the lenders' option, into either (a) preferred equity, or (b)
secured debt, no later than December 31, 1999, as described below. Hybridon will
pay the lenders interest monthly in arrears on the unpaid principal amount of
the loan at the rate of 8% per annum, payable in common stock at the rate of
$0.50 per share, on the first business day of each month that the loan is
outstanding, commencing November 1, 1999. The loan may be prepaid without
premium or penalty at any time. Any preferred stock into which such loan is
converted will (i) rank senior to existing preferred stock, but junior to all
debt, (ii) will be paid a dividend of 8% per annum, payable semi-annually in
arrears, which will be payable in Hybridon common stock, priced at the market
price on the record date, (iii) will be convertible to Hybridon common stock at
the rate of $0.50 per share at any time and (iv) will be callable by Hybridon at
any time after three years. Any secured debt into which such loan will be
converted will (a) have a five-year term, (b) will bear 8% interest, payable
semi-annually in arrears, payable in cash or Hybridon common stock, at
Hybridon's option, (c) will be convertible into common stock at $0.60 per share,
(d) will be prepayable by Hybridon, in whole or in part, at any time in cash;
provided however, that if the loan is prepaid at Hybridon's election during the
first three years of the term, Hybridon will issue a number of warrants with an
exercise price of $0.60 per share to purchase common stock

-29-

equal to the number of shares into which the amount prepaid was convertible, (e)
will be secured by all assets of Hybridon and (f) will rank pari passu with the
current $6.0 million loan held by the Lenders.

During October 1999, Hybridon commenced an offering that will extend
through December 1999. If such offering is consummated, the September notes and
October loans described above are expected to convert and become part of the
offering. The terms of the offering are as follows: (a) three-year term; (b)
interest rate of 8%, payable semi-annually in arrears; (c) interest is payable
in cash or in additional notes, at Hybridon's option; (d) convertible into
common stock at $0.60 per share; (e) prepayable by Hybridon, in whole or in
part, at any time in cash; (f) if prepaid at Hybridon's election during the
first three years of the term, Hybridon will issue a number of warrants to
purchase common stock equal to the number of shares into which the amount
prepaid was convertible, with a $0.60 strike price; and (g) secured by
substantially all assets. The securities offered have not been and will not be
registered under the Securities Act and may not be offered or sold in the U.S.
absent registration or an applicable exemption from registration requirements.
As of November 15, 1999, Hybridon had received approximately $500,000 (and an
additional $400,000 in escrow) under the terms of this offering. While the terms
of this financing have been agreed to, the parties have not yet finalized the
documentation. It is therefore possible that Hybridon may not consummate this
financing and gain use of these funds. Hybridon does not, however, anticipate
any such difficulties.

Hybridon's ability to continue operations in 1999 depends on its
success in obtaining new funds in the immediate future. Hybridon is currently
seeking debt or equity financing in an amount sufficient to support its
operations into 2000, and in connection therewith, is in negotiations with
several parties to obtain such financing. However, there can be no assurance
that Hybridon will obtain any funds or as to the timing thereof. Hybridon's
existing cash resources are expected to be sufficient to fund Hybridon's
operations through the end of 1999. If Hybridon is unable to obtain substantial
additional new funding by the end of 1999, Hybridon will be required to obtain
funds through arrangements with collaborative partners or others that may
require it to relinquish rights to certain of its technologies, product
candidates or products which it would otherwise pursue on its own, or terminate
operations or seek relief under applicable bankruptcy laws.

1998 FINANCING ACTIVITIES

On February 6, 1998, Hybridon commenced an offer to the holders of the
9% notes to exchange the 9% notes for Series A preferred stock and certain
warrants of Hybridon. On May 5, 1998, noteholders holding $48.7 million of
principal and $2.4 million of interest tendered such principal and accrued
interest to Hybridon for 510,505 shares of Series A preferred stock and warrants
to purchase 3,002,958 shares of common stock with an exercise price of $4.25 per
share.

On May 5, 1998, Hybridon completed a private offering of equity
securities raising total gross proceeds of approximately $26.7 million from the
issuance of 9,597,476 shares of common stock, 114,285 shares of Series A
preferred stock and warrants to purchase 3,329,486 shares of common stock at
$2.40 per share. The gross proceeds include the conversion of approximately $5.9
million of accounts payable, capital lease obligations and other obligations
into common stock. Hybridon incurred approximately $1.6 million of cash expenses
related to the private offering and issued 597,699 shares of common stock and
warrants to purchase 1,720,825 shares of common stock at $2.40 per share to the
placement agents. In addition, Hybridon is obligated to issue an additional
300,000 shares in connection with this transaction. For more information about
this transaction, see note 15(c) of the notes to consolidated statements.

Credit Facility

In December 1996, Hybridon entered into a five-year $7,500,000 note
payable with a bank. The note contained certain financial obligations that
required Hybridon to maintain a minimum worth and a minimum liquidity and
prohibited the payment of dividends. The note was payable in 59 equal
installments of $62,500 beginning on February 1, 1997, with a balloon payment of
the then remaining outstanding principal balance due on January 1, 2002. Because
Hybridon was required to make certain prepayments of principal during 1998, the
outstanding principal balance of the loan at November 16, 1998 was approximately
$2.8 million. The lender granted Hybridon a waiver of compliance with the
minimum worth requirement at December 31, 1998 and March 31, 1999 and the
minimum liquidity requirement at April 15, 1999.

-30-

Effective November 20, 1998, Forum Capital Markets, LLC and certain
investors associated with Pecks Management Partners Ltd. purchased the loan from
the bank. Forum and Pecks are affiliates of two members of Hybridon's board of
directors. In connection with this purchase, Forum and Pecks lent an additional
$3.2 million to Hybridon so as to increase the outstanding principal amount of
the note to $6,000,000. In addition, the terms of the note payable were amended
as follows:

o        the maturity was extended to November 30, 2003

o        the interest rate was decreased to 8%

o        interest is payable monthly in arrears, with the principal due in full
at maturity

o        the note payable is convertible, at the option of Forum and Pecks, in
whole or in part, into shares of common stock of Hybridon at a
conversion price equal to $2.40 a share

o        the threshold of the minimum liquidity obligation was reduced from
$4,000,000 to $2,000,000

o        the note payable may not be prepaid, in whole or in part, at any time
prior to December 1, 2000

The other terms of the note payable were unchanged.

For further information about this loan, see note 7 of the notes to
consolidated financial statements.

Facility Leases

As of December 31, 1998, Hybridon has future operating lease
commitments of approximately $7.7 million through 2007 for its existing leases.

Net Operating Loss Carryforwards

As of December 31, 1998, Hybridon had approximately $220.0 million and
$3.9 million of net operating loss and tax credit carryforwards, respectively.
The Tax Reform Act of 1986 contains certain provisions that may limit Hybridon's
ability to utilize net operating loss and tax credit carryforwards in any given
year if certain events occur, including cumulative changes in ownership
interests in excess of 50% over a three-year period. Hybridon has completed
several financings since the effective date of the Tax Act, which, as of
December 31, 1998, have resulted in ownership changes in excess of 50%, as
defined under the Tax Act and which will limit Hybridon's ability to utilize its
net operating loss carryforwards.

HISTORY OF OPERATING LOSSES; UNCERTAINTY OF FUTURE PROFITABILITY

Since inception, Hybridon has incurred significant losses, which it has
funded through the issuance of equity securities, debt issuances, sales by
Hybridon Specialty Products, and through research and development collaborations
and licensing arrangements.

FUTURE CAPITAL NEEDS; UNCERTAINTY OF ADDITIONAL FUNDING

Even though Hybridon has obtained sufficient cash to fund its
operations for the balance of 1999, it will be required to raise substantial
additional funds through external sources, including through collaborative
relationships and public or private financing, to support its operations
throughout 2000 and beyond . Except for research and development funding from
Searle under its collaborative agreement with Searle (which is subject to early
termination in certain circumstances), Hybridon has no committed external
sources of capital, and, as discussed above, expects no product revenues for
several years from sales of the therapeutic products that it is developing (as
opposed to sales of DNA products and reagents manufactured and sold by Hybridon
Specialty Products). No guarantee can be given that additional funds will be
available to fund operations for the balance of 1999 or in future years, or, if
available, that such funds will be available on acceptable terms. If additional
funds are raised by issuing equity securities, further dilution to then existing
stockholders will result. Additionally, the terms of any such additional
financing may adversely affect the holdings or rights of then existing
stockholders.

-31-

Hybridon's future capital requirements will depend on many factors,
including continued scientific progress in its research, drug discovery and
development programs, the magnitude of these programs, progress with preclinical
and clinical trials, sales of DNA products and reagents to third parties by
Hybridon Specialty Products and the margins on such sales, the time and costs
involved in obtaining regulatory approvals, the costs involved in filing,
prosecuting and enforcing patent claims, competing technological and market
developments, Hybridon's ability to establish and maintain collaborative
academic and commercial research, development and marketing relationships, its
ability to obtain third-party financing for leasehold improvements and other
capital expenditures and the costs of manufacturing scale-up and
commercialization activities and arrangements.

DIRECTORS AND EXECUTIVE OFFICERS
OF HYBRIDON

The following table sets forth certain information regarding the
executive officers and directors of Hybridon as of February 15, 2000.

Name                                          Age        Position
----                                          ---        --------

E. Andrews Grinstead, III.................    54         Director  (Class  III),   President  and  Chief  Executive
Officer
Sudhir Agrawal, D. Phil...................    45         President and Acting Chief Executive  Officer,  
Senior Vice President of Discovery,  Chief Scientific Officer,  
and Director (Class III)
James B. Wyngaarden, M.D..................    73         Chairman of the Board of Directors (Class II)
Nasser Menhall............................    42         Director (Class I)
Arthur W. Berry...........................    56         Director (Class I)
Harold L. Purkey..........................    54         Director (Class I)
Paul C. Zamecnik, M.D.....................    85         Director (Class II)
Camille Chebeir...........................    60         Director (Class II)
Youssef El-Zein...........................    49         Director (Class III) 

E. Andrews Grinstead, III joined Hybridon in June 1991 and was
appointed Chairman of the board and Chief Executive Officer in August 1991 and
President in January 1993. He has served on the board of directors since June
1991. Mr. Grinstead resigned as Chairman in December 1999. On February 15, 2000,
Hybridon announced that Mr. Grinstead had taken an unexpected medical leave of
absence of indefinite duration due to a serious illness and that Mr. Grinstend
had been replaced as President. Prior to joining Hybridon, Mr. Grinstead served
as Managing Director and Group Head of the life sciences group at Paine Webber,
Incorporated, an investment banking firm, from 1987 to October 1990; Managing
Director and Group Head of the life sciences group at Drexel Burnham Lambert,
Inc., an investment banking firm, from 1986 to 1987; and Vice President at
Kidder, Peabody & Co. Incorporated, an investment banking firm, from 1984 to
1986, where he developed the life sciences corporate finance specialty group.
Mr. Grinstead served in a variety of operational and executive positions with
Eli Lilly and Company, an international pharmaceutical company, from 1976 to
1984, most recently as General Manager of Venezuelan Pharmaceutical, Animal
Health and Agricultural Chemical Operations and at Eli Lilly Corporate Staff as
Administrator, Strategic Planning and Acquisitions. Since 1991, Mr. Grinstead
has served as a director of Pharmos Corporation, a development stage company
engaged in the development of novel pharmaceutical compounds and drug delivery
systems. Mr. Grinstead also serves as a director of Meridian Medical
Technologies, Inc., a pharmaceutical and medical device company. Mr. Grinstead
was appointed to The President's Council of the National Academy of Sciences and
the Institute of Medicine in January 1992 and the board of the Massachusetts
Biotech Council in 1997. Since 1994, Mr. Grinstead has served as a member of the
board of trustees of the Albert B. Sabin Vaccine Foundation, a charitable
foundation dedicated to disease prevention. Mr. Grinstead received an A.B. from
Harvard College in 1967, a J.D. from the University of Virginia School of Law in
1974 and an M.B.A. from the Harvard Graduate School of Business Administration
in 1976.

Sudhir Agrawal joined Hybridon in February 1990 and served as Principal
Research Scientist from February 1990 to January 1993 and as Vice President of
Discovery from December 1991 to January 1993 prior to being appointed Chief
Scientific Officer in January 1993, Senior Vice President of Discovery in March
1994, and President and Acting Chief Executive Officer in February
2000. He has served on the board of directors since March 1993. Prior to joining

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Hybridon, Dr. Agrawal served as a Foundation Scholar at the Worcester Foundation
from 1987 through 1991. Dr. Agrawal served as a Research Associate at Research
Council Laboratory of Molecular Biology in Cambridge, England from 1985 to 1986,
studying synthetic oligonucleotides. Dr. Agrawal received a B.Sc. in chemistry,
botany and zoology in 1973, an M.Sc. in organic chemistry in 1975 and a D. Phil.
in chemistry in 1980 from Allahabad University in India.

James B. Wyngaarden was appointed member of the board of directors of
Hybridon in 1990, was Vice Chairman of the board of directors of Hybridon from
February 1997 to February 2000, and in February 2000 was appointed Chairman of
the board of directors of Hybridon. He was Foreign Secretary of the National
Academy of Sciences and the Institute of Medicine of the National Academy of
Sciences from 1990 to 1994; council member of the Human Genome Organization from
1990 to 1993 and Director from 1990 to 1991; and Director of the National
Institutes of Health from 1982 to 1989. He is a member of the board of directors
of Human Genome Sciences, Inc. and Magainin Pharmaceuticals, Inc.

Nasser Menhall was appointed member of the board of directors of
Hybridon in 1992. He has been a member of the board of directors and Chief
Executive Officer of the WorldCare Group, a teleradiology company, since 1993;
President of Pillar Limited, a private investment and management consulting
firm, since 1990; and President of Biomedical Associates, a private investment
firm, since 1990.

Arthur W. Berry was appointed member of the board of directors of
Hybridon in 1998. He has been Chairman and Managing Partner of Pecks Management
Partners, since 1990, and was Vice President and Co-Manager of the Alliance
Convertible Securities Group and President of the Alliance Convertible Fund from
1985 to 1990. Prior to joining Alliance, he was Vice President and Head of
Special Funds Section and Manager of the Harris Convertible Fund at Harris Bank
and Senior Portfolio Manager in the bank's Individual Investment Management
Group. He is also a member of the board of directors of Intellicorp, Inc.

Harold L. Purkey was appointed member of the board of directors of
Hybridon in 1998. He is President of Forum Capital Markets LLC, and was
previously Senior Managing Director of convertible securities at Smith Barney
Shearson from 1990 to 1994, and Senior Executive Vice President of Drexel
Burnham Lambert from 1982 to 1989. He is also a member of the board of directors
of Richardson Electronics.

Paul C. Zamecnik was appointed member of the board of directors of
Hybridon in 1990. He was Principal Scientist at the Worcester Foundation for
Biomedical Research, Inc. from 1979 to 1996, and has been Collis P. Huntington
Professor of Oncologic Medicine Emeritus at the Harvard Medical School since
1979. He is also currently Senior Scientist and Honorary Physician at
Massachusetts General Hospital in Boston.

Youssef El-Zein was appointed member of the board of directors of
Hybridon in 1992, and has been Vice Chairman of the board of directors of
Hybridon since February 1997. He has been Executive Officer of Pillar S.A., a
private investment and management consulting firm, since 1991; Chairman of the
WorldCare Group since 1993; and member of the board of directors of Pillar
Investment Limited ("Pillar Investment"), a private investment and management
consulting firm, since 1991.

Camille Chebeir was appointed member of the board of directors of
Hybridon in 1999. Since 1995, he has been President of Sedco Services, Inc., a
company which manages investments of the bin Mafouz Saudi Arabian family. In
that capacity, he serves on the boards of various entities in which Sedco
Services, Inc. invests. Mr. Chebeir was previously the Executive Vice
President/General Manager of National Commercial Bank, New York branch. Mr.
Chebeir is a former President of the Arab Bankers Association of North America.

Hybridon's restated certificate of incorporation provides for a
staggered board of directors consisting of three classes, with each class being
as nearly equal in number as possible. At each annual meeting of Hybridon's
stockholders, the term of one class ends and the successors of the directors in
that class are elected for a term of three years. Hybridon has designated three
Class I directors, three Class II directors, and four Class III directors; they
are identified in the above table. They are to serve until the annual meeting of
stockholders to be held in 2000, 2001 and 2002, respectively, and until their
respective successors are elected and qualified, or until their earlier
resignation or removal. The restated certificate of incorporation provides that
directors may be removed only for cause by a majority of stockholders.

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EXECUTIVE COMPENSATION

COMPENSATION OF EXECUTIVE OFFICERS

Summary Compensation Table

The following table sets forth the compensation for the fiscal years
ended December 31, 1999 ("fiscal 1999"), December 31, 1998 and December 31, 1997
for Hybridon's Chief Executive Officer and Chief Scientific Officer, who were
serving as Executive Officers at December 31, 1999 and whose total annual salary
and bonus exceeded $100,000 in fiscal 1999:

SUMMARY COMPENSATION TABLE

LONG-TERM
ANNUAL COMPENSATION                COMPENSATION
-----------------------------------------------------   AWARDS
OTHER
NAME AND PRINCIPAL POSITION                                               ANNUAL      SECURITIES        
---------------------------                                               COMPEN-     UNDERLYING      ALL OTHER 
SALARY          BONUS    SATION       OPTIONS       COMPENSATION
------          -----    ------       -------       ------------

E. Andrews Grinstead, III ...........   1999   $ 375,000           0   $  93,750(1)   1,763,319(3)   $  42,548(2)
Chief Executive  Officer             1998   $ 375,000           0   $  93,750(1)     500,000      $  44,832(2)
and Director                         1997   $ 375,000           0   $  93,750(1)      66,806      $  53,784(2)

Sudhir Agrawal, D. Phil .............   1999   $ 250,000           0   $  50,000(1)   1,618,263(3)   $  25,962(2)
President, Acting Chief Executive .   1998   $ 250,000           0   $  50,000(1)     500,000      $  22,115(2)
Senior Vice President of              1997   $ 250,000           0   $  50,000(1)      32,263      $  13,462(2)
Discovery, and Chief Scientific
Officer and Director

----------
(1)      Other annual compensation paid, or to be paid, by Hybridon to, or for
the benefit of, the named executive officers is as follows:

E. Andrews Grinstead, III                1999     1998      1997
-------------------------                ----     ----      ----

Paid in lieu of employee benefits ..   $79,288   $79,903   $34,902
Purchase of life insurance and other
payments to third parties ........    14,462    13,487    58,848
Total ..............................   $93,750   $93,750   $93,750

Sudhir Agrawal, D. Phil                  1999     1998      1997
-----------------------                  ----     ----      ----

Paid in lieu of employee benefits ..   $36,789   $37,462   $38,132
Purchase of life insurance and other     
payments to third parties              13,211    12,538    11,868
Total ..............................   $50,000   $50,000   $50,000

(2)      All other compensation paid, or to be paid, by Hybridon to, or for the
benefit of, the named executive officers is as follows:

E. Andrews Grinstead, III               1999     1998      1997
-------------------------               ----     ----      ----

Surrender of unused vacation days ..   $42,548   $28,832   $37,300
Additional payments ................         0    16,000    16,484

Total ..............................   $42,548   $44,832   $53,784

Sudhir Agrawal, D. Phil                 1999     1998      1997
-----------------------                 ----     ----      ----

Surrender of unused vacation days ..   $25,962   $22,115   $13,462

Total ..............................   $25,962   $22,115   $13,462

(3)  During 1999 Hybridon reduced the exercise price of all employee stock
options to $.50 per share. The number of repriced stock options amounts to
1,263,319 and 1,118,263 for Mr. Grinstead and Dr. Agrawal, respectively.
These repriced stock options are included in the "Summary Compensation
Table."

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Option Grants and Repricings Table

The following table sets forth certain information concerning grants
and repricings of stock options made during fiscal 1999 to each of the named
executive officers:

OPTION GRANTS AND REPRICINGS IN LAST FISCAL YEAR

INDIVIDUAL GRANTS

POTENTIAL REALIZABLE
PERCENTAGE                                       VALUE AT ASSUMED
OF TOTAL                                      ANNUAL RATES OF STOCK
NUMBER OF         OPTIONS                                     PRICE APPRECIATION FOR
SECURITIES       GRANTED TO       EXERCISE                        OPTIONS TERM(2) 
UNDERLYING      EMPLOYEES IN       PRICE
OPTIONS        FISCAL YEAR        PER         EXPIRATION
GRANTED                         SHARE        DATE(1)           5%       10%

E. Andrews Grinstead, III         

01/01/99 grant                     500,000              7.7%          $2.00       0/1/01/09      $323,477     $1,107,416
1999 repricings                  1,263,319             19.4%          $0.50        Various       $ 79,821     $  423,299
Total granted or repriced in     1,763,319
1999
Less duplication for options
granted and repriced in        (500,000)
1999
Total options outstanding at
12/31/99                      1,263,319

Sudhir Agrawal, D.Phil.
01/01/99 grant                     500,000              7.7%          $2.00       0/1/01/09      $323,477     $1,107,416
1999 repricings                  1,118,263             17.2%          $0.50        Various       $ 82,267     $  405,914
Total granted or repriced in     1,618,263
1999
Less duplication for options      (500,000)
granted and repriced in
1999
Total options outstanding at     1,118,263
12/31/99

---------------

(1)      The expiration date of each option is the tenth anniversary of the date
on which the option was originally granted.

(2)      The amounts shown on this table represent hypothetical gains that could
be achieved for the respective options if exercised at the end of the
option term. These gains are based on assumed rates of stock increase
of 5% and 10%, compounded annually from the date the respective options
were repriced or granted to their expiration date. The gains shown are
net of the option exercise price, but do not include deductions for
taxes or other expenses associated with the exercise. Actual gains, if
any, on stock option exercises will depend on the future performance of
the common stock, the optionholder's continued employment through the
option period, and the date on which the options are exercised. As of
February 2, 2000, the last sale price of common stock of Hybridon was
$1.75.

(3)      Mr. Grinstead and Dr. Agrawal had 680,596 and 551,356 exercisable
options, respectively, at 12/31/99. The remaining options become
exercisable over various periods through 9/30/03.

-35-

Stock Option Repricing

The following table sets forth all repricings of stock options held by
E. Andrews Grinstead, III, Hybridon's Chief Executive Officer, and Sudhir
Agrawal, Hybridon's President and Acting Chief Executive Officer, since
Hybridon's initial public offering on February 2, 1996.

10-YEAR OPTION/SAR REPRICINGS

NUMBER OF                                                              LENGTH OF
SECURITIES     MARKET PRICE                                         ORIGINAL OPTION
UNDERLYING      OF STOCK AT          EXERCISE                        TERM REMAINING
OPTIONS/SARS'       TIME OF         PRICE AT TIME         NEW            AT DATE OF
DATE          REPRICED OR     REPRICING OR        OF REPRICING       EXERCISE        REPRICING OR
AMENDED         AMENDMENT         OR AMENDMENT        PRICE           AMENDMENT

E. Andrews Grinstead, III

09/23/99      500,000            $0.38                $2.00            $0.50              9.28
09/23/99      500,000            $0.38                $2.00            $0.50              8.83
09/23/99       12,000            $0.38               $31.88            $0.50              7.66
09/23/99       38,000            $0.38               $30.00            $0.50              7.54
09/23/99       16,806            $0.38               $31.25            $0.50              7.41
09/23/99       50,000            $0.38               $57.85            $0.50              6.42
09/23/99       30,000            $0.38               $37.50            $0.50              5.48
09/23/99       19,600            $0.38               $37.50            $0.50              3.96
09/23/99       70,246            $0.38               $37.50            $0.50              3.62
09/23/99       26,667            $0.38               $25.00            $0.50              2.38

Sudhir Agrawal, D.Phil.
09/23/99      500,000            $0.38                $2.00            $0.50              9.28
09/23/99      500,000            $0.38                $2.00            $0.50              8.83
09/23/99        6,000            $0.38               $31.88            $0.50              7.66
09/23/99       19,000            $0.38               $30.00            $0.50              7.54
09/23/99        7,263            $0.38               $31.25            $0.50              7.41
09/23/99       25,000            $0.38               $57.85            $0.50              6.42
09/23/99       20,000            $0.38               $37.50            $0.50              5.48
09/23/99       10,000            $0.38               $37.50            $0.50              3.29
09/23/99       21,000            $0.38               $17.50            $0.50              3.29
09/23/99       10,000            $0.38                $1.25            $0.50              2.38

The board of directors repriced all employee stock options effective September
23, 1999. The options were repriced in order to provide additional incentives to
employees, since the previous option exercise prices were greater than the
market price of Hybridon's common stock.

Aggregated Option Exercises and Year-End Option Table

The following table sets forth certain information concerning the
number and value of unexercised options held by each of the named executive
officers on December 31, 1999:

-36-

AGGREGATED OPTION EXERCISES IN LAST
FISCAL YEAR AND FISCAL YEAR-END
OPTION VALUES

NUMBER OF              VALUE OF
SHARES              UNEXERCISED
UNDERLYING           IN THE MONEY
OPTIONS AT         OPTIONS AT FISCAL
FISCAL YEAR-END         YEAR- END(1)
EXERCISABLE/          EXERCISABLE/
UNEXERCISABLE          UNEXERCISABLE

E. Andrews Grinstead, III............   680,596 / 582,723    $347,104 / $297,189
Sudhir Agrawal.......................   551,356 / 566,907    $281,192 / $289,123

----------
(1)      The closing price for the common stock as reported by The Nasdaq OTC
Bulletin Board on December 31, 1999 was $1.01. Value is calculated on
the basis of the difference between the option exercise price and
$1.01, multiplied by the number of shares of common stock underlying
the option.

DIRECTOR COMPENSATION

Each non-employee director is paid $1,500 for personal or telephonic
attendance at a board of directors or committee meeting. Other directors are not
entitled to compensation in their capacities as directors. All of the directors
are reimbursed for their expenses incurred in connection with their attendance
at board of directors and committee meetings. In addition, Dr. Zamecnik received
compensation in the amount of $83,995 in 1998 and $26,000 in 1999 in connection
with certain consulting services to Hybridon. Of this amount, Dr. Zamecnik
received 25,000 shares of common stock and warrants to purchase 6,250 shares of
common stock in lieu of $50,000 in cash, and $26,000 in convertible debt in lieu
of $26,000 in cash, which is convertible, at Dr. Zamecnik's option, into 43,333
shares of common stock. The remaining $33,995 was paid in cash. Hybridon also is
a party to consulting, advisory and other arrangements with various directors
and their affiliates. For a description of the foregoing arrangements with
Hybridon and certain other transactions between Hybridon and affiliates of
certain directors, see "Certain Transactions."

In October 1995, Hybridon adopted the 1995 director stock option plan.
Under the terms of the director plan, options to purchase 1,000 shares of common
stock were granted to each director of Hybridon, other than Mr. Grinstead and
Dr. Agrawal, (a) as of January 24, 1996 at an exercise price of $65.625 per
share, (b) as of May 1, 1997, at an exercise price of $27.50 per share, (c) as
of May 1, 1998 at an exercise price of $2.375 per share, and (d) as of May 1,
1999 at an exercise price of $1.22 per share. The director plan also provides
that options to purchase 5,000 shares of common stock will be granted to each
new director upon his or her initial election to the board of directors.
However, because of the one-for-five reverse stock split described below,
options to purchase 1,000 shares of common stock were granted to Camille Chebeir
and H.F. (Jake) Powell upon their appointment to the board of directors in 1999.
(Mr. Powell has since resigned from Hybridon's board of directors.) In addition,
on June 8, 1999, Hybridon's stockholders approved a one-time grant of options to
purchase 8,000 shares of Hybridon's common stock at an exercise price of $0.47
per share to each director other than Mr. Grinstead and Dr. Agrawal. Annual
options to purchase 5,000 shares of common stock will be granted to each
eligible director on May 1 of each year. All options will vest on the first
anniversary of the date of grant or, in the case of options granted
automatically each year, on April 30 of the year following the date of the
grant; provided, that the exercisability of these options will be accelerated
upon the occurrence of a change in control, as defined in the director plan. A
total of 400,000 shares of common stock may be issued upon the exercise of stock
options granted under the director plan. The exercise price of options granted
under the director plan will equal the closing price of the common stock on the
date of grant. As of June 15, 1999, options to purchase an aggregate of 93,000
shares of common stock were outstanding under the director plan.

Non-employee directors also have received options to purchase common
stock of Hybridon under Hybridon's 1997 stock incentive plan and Hybridon's 1995
stock option plan. In particular, in 1998, the board of directors voted to grant
an option to purchase 50,000 shares of common stock at $2.00 per share to Dr.
Wyngaarden and Mr. El-Zein, in recognition of their services as Vice Chairmen of
the board of directors during the previous

-37-

twelve months. Mr. El-Zein declined this grant. In addition, in 1998, the board
of directors voted to grant 50,000 shares of common stock of Hybridon to Dr.
Zamecnik in recognition of his outstanding contributions to Hybridon.

Employment Agreements, Termination of Employment and Change in Control
Arrangements

Hybridon is party to an employment agreement with Mr. Grinstead for the
period commencing July 1, 1996 and ending June 30, 2001. Under this agreement,
Mr. Grinstead is currently entitled to receive an annual base salary of
$375,000. Mr. Grinstead also is eligible to receive (i) a cash bonus each year
related to the attainment of management objectives specified by the board of
directors and (ii) additional payments of $16,000 in 1996, 1997 and 1998. In the
event Mr. Grinstead's employment is terminated by Hybridon without cause or by
him for good cause, Hybridon will pay Mr. Grinstead during the 24-month period
following his termination a monthly amount equal to one-twelfth of the sum of
Mr. Grinstead's annual base salary as of the date of termination and the average
bonus paid to him during the three years preceding his termination. Hybridon
also will continue Mr. Grinstead's benefits for such period, subject to earlier
termination under certain circumstances. If his employment is terminated by
Hybridon for failure to perform his assigned duties, he will continue to receive
his annual base salary and benefits during the six-month period following such
termination. Notwithstanding the foregoing, in the event that Mr. Grinstead's
employment is terminated for any of the above reasons within 12 months following
a change in control of Hybridon, Mr. Grinstead will be entitled to receive, in
lieu of the payments described above, a lump sum payment equal to 300% of the
sum of his annual base salary and his average bonus amount.

On February 15, 2000, Hybridon announced that Mr. Grinstead had taken
an unexpected medical leave of absence of indefinite duration due to a serious
illness. Mr. Grinstead's employment agreement remains in effect.

In accordance with the terms of Mr. Grinstead's previous employment
agreement, Hybridon loaned $190,000 to Mr. Grinstead in December 1992 pursuant
to the terms of a promissory note bearing simple interest at a rate of 6% per
year, which originally provided for the payment of principal and all interest on
the earlier of December 23, 1995 or the expiration or termination of Mr.
Grinstead's employment by Hybridon, but is currently payable on demand. This
loan remained outstanding as of December 31, 1999, at which date the total
unpaid balance of principal and interest was $270,050.

Hybridon is party to an employment agreement with Dr. Agrawal for the
period beginning July 1, 1996 and ending June 30, 2000. Under this agreement,
Dr. Agrawal serves as Senior Vice President of Discovery and Chief Scientific
Officer of Hybridon and is currently entitled to receive an annual base salary
of $250,000. When Dr. Agrawal was appointed President and Acting Chief Executive
Officer on February 14, 2000, the terms of his employment remained unchanged.
Dr. Agrawal is eligible to receive a cash bonus each year for achieving
management objectives specified by the Chief Executive Officer and the board of
directors. In the event Dr. Agrawal's employment is terminated by Hybridon
without cause or by him for good cause, Hybridon will pay Dr. Agrawal during the
24-month period following his termination a monthly amount equal to one-twelfth
of the sum of Dr. Agrawal's annual base salary as of the date of termination and
the average bonus paid to him during the three years preceding his termination.
Hybridon will also continue Dr. Agrawal's benefits for such period, subject to
earlier termination under certain circumstances. If his employment is terminated
by Hybridon for failure to perform his assigned duties, he will continue to
receive his annual base salary and benefits during the six-month period
following such termination. Notwithstanding the foregoing, in the event that Dr.
Agrawal's employment is terminated for any of the above reasons within 12 months
following a change in control of Hybridon, Dr. Agrawal will be entitled to
receive, in lieu of the payments described above, a lump sum payment equal to
300% of the sum of his annual base salary and his average bonus amount.

The employment agreements entered into between Hybridon and each of Mr.
Grinstead and Dr. Agrawal also provide that all stock options held by any of the
Named Executive Officers, including existing options and options to be granted
in the future, shall include terms providing (i) that in the event that such
Named Executive Officer's employment is terminated by Hybridon without cause or
by him for good cause the exercisability of such stock options will be
accelerated by two years and such stock options will be exercisable for a
two-year period following termination and (ii) that in the event of certain
changes in control of Hybridon, its liquidation or the sale of all or
substantially all of its assets, all such stock options not then exercisable
will vest and become immediately exercisable. Hybridon is also a party to
registration rights agreements with Mr. Grinstead that provide that in the event
Hybridon proposes to register any of its securities under the Securities Act, at
any time, with certain exceptions, Mr. Grinstead shall be entitled to include
the shares of common stock held by him in such registration, subject to the
right of the managing underwriter of any underwritten offering to exclude from
such registration for marketing reasons some or all of such shares. 

-38-

Hybridon also is a party to indemnification agreements with Mr. Grinstead
pursuant to which Hybridon has agreed to indemnify him for certain liabilities,
including liabilities arising under the Securities Act.

Stock options to purchase an aggregate of 207,513 shares of common
stock granted to the Named Executive Officers pursuant to the 1990 Plan provide
that, upon a change in control, all options granted thereunder will become fully
exercisable. In addition, pursuant to the terms of the employment agreements
entered into between Hybridon and each of the Named Executive Officers described
above (i) in April 1997, stock options to purchase an aggregate of 156,069
shares of common stock granted to the Named Executive Officers under Hybridon's
1995 plan were amended to provide that such options will become fully
exercisable upon a change in control of Hybridon, and (ii) all stock options
granted to the Named Executive Officers after March 1, 1997 will provide that
such options will become fully exercisable upon a change of control of Hybridon.

COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION

On June 16, 1998 the board of directors re-established a Compensation
Committee consisting of Messrs. Berry and El-Zein and Dr. Wyngaarden. None of
the directors or executive officers of Hybridon had any "interlock"
relationships to report during Hybridon's fiscal year ended December 31, 1999.

Since January 1, 1999, Hybridon has entered into or is involved in
certain ongoing transactions with (i) Pillar S.A., Pillar Investment, Pillar
Limited and Charles River Building Limited Partnership, entities of which
Messrs. El-Zein and Menhall are affiliates; (ii) entities advised by Pecks, an
entity of which Mr. Berry is a principal; (iii) Forum, an entity of which Mr.
Purkey is an affiliate; and (iv) each of Drs. Wyngaarden and Zamecnik and Mr.
Powell. See "Certain Transactions."

-39-

SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

The following table sets forth certain information as of December 31,
1999 with respect to the beneficial ownership of shares of common stock by each
person known to Hybridon to own beneficially more than 5% of the outstanding
shares of common stock, assuming conversion of all convertible debt or preferred
stock and exercise of all warrants and stock options by such person and only by
such person.

Amount and Nature
of Beneficial Ownership(1)
Name and Address of                                    Number of                 Percent of
Beneficial Owner                                        Shares                     Class
----------------                                        ------                     -----

5% STOCKHOLDERS

Forum Capital Markets LLC...................         6,083,394(2)                  28.23%
53 Forest Ave.
Old Greenwich, CT  06870

Pecks Management Partners Ltd...............
One Rockefeller Plaza                                4,160,048(3)                  20.37%
New York, New York  10022

General Motors Employees....................
Domestic Group Trust                                 3,832,220(4)                  19.07%
c/o Pecks Management Partners Ltd.
One Rockefeller Plaza
New York, New York  10020

E. Andrews Grinstead, III...................
Hybridon, Inc.                                       3,344,843(5)                  17.10%
155 Fortune Blvd.
Milford, MA  01757

Guardian Life Insurance.....................         3,255,110(6)                  16.68%
Company of America
201 Park Avenue South, 7A
New York, New York  10003

Youssef El-Zein.............................         2,692,339(7)                  14.56%
28 Avenue de Messine
75008 Paris, France

Nasser Menhall..............................         2,670,351(8)                  14.46%
28 Avenue de Messine
75008 Paris, France

Pillar Investment Limited...................         2,560,356(9)                  13.94%
28 Avenue de Messine
75008 Paris, France

Intercity Holdings Ltd......................         2,216,666(10)                 13.32%
c/o Cuson Milner House
18 Parliament Street
Hamilton, Bermuda

Abdelah Bin Mahfouz.........................         2,216,666(11)                 13.32%
c/o SEDCO
P.O. Box 4384
Jeddah 21491
Saudi Arabia

Delaware State Employees....................         2,549,833(12)                 13.55%
Retirement Fund
c/o Pecks Management Partners Ltd.
One Rockefeller Plaza
New York, New York  10020

-40-

Amount and Nature
of Beneficial Ownership(1)
Name and Address of                                    Number of                 Percent of
Beneficial Owner                                        Shares                     Class
----------------                                        ------                     -----

Yahia M. A. Bin Laden.......................         1,373,977(13)                 8.33%
2 rue Charles Bonnet
1206 Geneva, Switzerland

Nicris Limited..............................         1,360,644(14)                 8.25%
c/o Magnin Dunand & Associates
2 rue Charles Bonnet
1206 Geneva, Switzerland
1,317,755(15)                 7.72%

Darrier Hentsch & Cie.......................
4, rue de Saussure
1204 Geneva, Switzerland                             1,279,717(16)                 7.29%

Lincoln National Life Insurance Co..........
c/o Lynch & Mayer
520 Madison Avenue
New York, New York  10022
1,043,112(17)                 6.32%

Faisal Finance Switzerland SA...............
84 Ave Louis Casi
1216 Geneva, Switzerland

Finova Technology Finance Inc. .............         896,875 (18)                  5.43%
10 Waterside Drive
Farmington, CT  06032

Declaration of Trust for the................          924,456(19)                  5.38%
Defined Benefit Plan of ICI
American Holdings, Inc.
c/o Pecks Management Partners Ltd.
One Rockefeller Plaza
New York, New York  10022

(1)       The number of shares beneficially owned is determined under rules
promulgated by the Securities and Exchange Commission, and the
information is not necessarily indicative of beneficial ownership for
any other purpose. Under these rules, beneficial ownership includes
any shares as to which the individual has sole or shared voting power
or investment power and also any shares which the individual has the
right to acquire within 60 days after December 31, 1999 through the
exercise of any stock option or other right. The inclusion herein of
such shares, however, does not constitute an admission that the named
stockholder is a direct or indirect beneficial owner of such shares.
Unless otherwise indicated, each person or entity named in the table
has sole voting power and investment power, or shares such power with
his or her spouse, with respect to all shares of capital stock listed
as owned by such person or entity.

(2)       Includes (a) 328,677 shares issuable upon exercise of Class B
warrants, (b) 280,517 shares issuable upon the exercise of Class C
warrants, (c) 468,859 shares issuable upon exercise of Class A
warrants, (d) 25,812 shares issuable upon the exercise of Class D
warrants, (e) 761,568 shares issuable upon exercise of other warrants,
(f) 1,250,000 shares issuable upon conversion of Forum's portion of
the $6,000,000 bank loan to Hybridon, and (g) 1,755,035 shares
issuable upon conversion of 74,589 shares of Series A preferred stock
owned by Forum and (h) 416,667 shares issuable upon conversion of
$250,000 in convertible debt.

(3)       Includes 122,078 shares of Series A preferred stock owned by four
investment advisory clients of Pecks, which clients would receive
dividends and the proceeds from the sale of such shares. Two of these
clients are Delaware State Employees Retirement Fund and Declaration
of Trust for the Defined Benefit Plan of ICI American Holdings, Inc.
These shares of Series A preferred stock are convertible into
2,872,424 shares of common stock of Hybridon. This amount also
includes 208,895 shares issuable upon the exercise of Class A warrants
and 394,354 shares issuable upon the exercise of Class D warrants held
in the total by the foregoing entities. This number also includes
684,375 shares issuable upon conversion of a portion of the $6,000,000
bank loan to Hybridon owned by certain of the foregoing entities.

-41-

(4)       Includes 117,887 shares of Series A preferred stock which are
convertible into 2,773,812 shares of common stock of Hybridon. This
amount also includes 492,783 shares issuable upon the exercise of
Class A warrants and 565,625 shares issuable upon conversion of a
portion of a $6,000,000 bank loan to Hybridon owned by this entity.

(5)       Includes 730,596 shares subject to outstanding stock options are
exercisable within the 60 day period following December 31, 1999, as
well as 2,566,667 shares issuable upon the conversion of $1,540,000 in
convertible debt owed by Mr. Grinstead.

(6)       Includes 112,612 shares of Series A preferred stock which are
convertible into 2,649,694 shares of common stock of Hybridon. This
amount also includes 353,316 shares issuable upon the exercise of
Class A warrants and 252,100 shares issuable upon the exercise of
Class D warrants.

(7)       Includes (a) 82,183 shares issuable upon the exercise of warrants held
by Mr. El-Zein, (b) 366 shares issuable upon the exercise of warrants
held by Pillar Associated, (c) 20,000 shares issuable upon the
exercise of warrants held by Pillar S.A., (d) 20,000 shares issuable
upon the exercise of warrants held by Pillar S.A.R.L., (e) 37,500
shares issuable upon the exercise of Class C warrants held by Pillar
Investment Limited, (f) 473,598 issuable upon the exercise of advisory
warrants held by Pillar Investment Limited, (g) 638,032 shares
issuable upon the exercise of placement warrants held by Pillar
Investment Limited, (h) 5,243 shares issuable upon the exercise of
other warrants held by Pillar Investment Limited, (i) 462,800 shares
held by Pillar Investment Limited, (j) 9,000 shares issuable upon the
exercise of stock options held by Mr. El-Zein, (k) 447,150 shares
issuable upon the conversion of $269,290 in convertible debt to be
issued to Pillar Investment Limited and (l) 496,833 shares issuable
upon the conversion of $298,100 in convertible debt that Pillar
Investment Limited has the right to acquire upon exercise of warrants.
Receipt by Pillar Investment Limited of the securities described in
(k) and (l) is subject to receipt by Hybridon of a fairness opinion.
Mr. El-Zein, an affiliate of Pillar Associated, Pillar S.A., Pillar
S.A.R.L. and Pillar Investment Limited, may be considered a beneficial
owner of the shares beneficially owned by such entities.

(8)       Includes (a) 60,195 shares issuable upon the exercise of warrants held
by Mr. Menhall, (b) 366 shares issuable upon the exercise of warrants
held by Pillar Associated, (c) 20,000 shares issuable upon the
exercise of warrants held by Pillar S.A., (d) 20,000 shares issuable
upon the exercise of warrants held by Pillar S.A.R.L., (e) 37,500
shares issuable upon the exercise of Class C warrants held by Pillar
Investment Limited, (f) 473,598 issuable upon the exercise of advisory
warrants held by Pillar Investment Limited, (g) 638,032 shares
issuable upon the exercise of placement warrants held by Pillar
Investment Limited, (h) 5,243 shares issuable upon the exercise of
other warrants held by Pillar Investment Limited, (i) 462,800 shares
held by Pillar Investment Limited, (j) 9,000 shares issuable upon the
exercise of stock options held by Mr. Menhall, (k) 447,150 shares
issuable upon the conversion of $269,290 in convertible debt to be
issued to Pillar Investment Limited and (l) 496,833 shares issuable
upon the conversion of $298,100 in convertible debt that Pillar
Investment Limited has the right to acquire upon exercise of warrants.
Receipt by Pillar Investment Limited of the securities described in
(k) and (l) is subject to receipt by Hybridon of a fairness opinion.
Mr. Menhall, an affiliate of Pillar Associated, Pillar S.A., Pillar
S.A.R.L. and Pillar Investment Limited, may be considered a beneficial
owner of the shares beneficially owned by such entities.

(9)       Includes (a) 37,500 shares issuable upon the exercise of Class C
warrants held by Pillar Investment Limited, (c) 473,598 issuable upon
the exercise of advisory warrants held by Pillar Investment Limited,
(c) 638,032 shares issuable upon the exercise of placement warrants
held by Pillar Investment Limited, (d) 5,243 shares issuable upon the
exercise of other warrants held by Pillar Investment Limited, (e)
447,150 shares issuable upon the conversion of $269,290 in convertible
debt to be issued to Pillar Investment Limited and (f) 496,833 shares
issuable upon the conversion of $298,100 in convertible debt that
Pillar Investment Limited has the right to acquire upon exercise of
warrants. Receipt by Pillar Investment Limited of the securities
described in (e) and (f) is subject to the receipt by Hybridon of a
fairness opinion.

(10)      Includes 375,000 shares issuable upon the exercise of Class B warrants
held by Intercity Holdings Ltd.

-42-

(11)      Includes 1,841,666 shares held by Intercity Holdings Ltd. and 375,000
shares issuable upon exercise of Class B warrants held by Intercity
Holdings. Mr. Mahfouz, a controlling stockholder of Intercity Holdings
Ltd., may be considered a beneficial owner of the shares beneficially
owned by such entity.

(12)      Includes 75,926 shares of Series A preferred stock which are
convertible into 1,786,494 shares of common stock of Hybridon. This
amount also includes 137,918 shares issuable upon the exercise of
Class A warrants, 270,271 shares issuable upon the exercise of Class D
warrants and 355,250 shares issuable upon conversion of portion of the
$6,000,000 bank loan to Hybridon owned by this entity.

(13)      Includes 1,125,880 shares held by Nicris Limited and 234,764 shares
issuable upon the exercise of Class B warrants held by Nicris Limited.
Mr. Bin Laden, a controlling stockholder of Nicris, may be considered
a beneficial owner of the shares beneficially owned by such entity.

(14)      Includes 234,764 shares issuable upon the exercise of Class B warrants
held by Nicris Limited.

(15)      Includes 143,636 shares issuable upon the exercise of Class B warrants
held by Darrier Hentsch and 666,667 shares issuable upon the
conversion of $400,000 in convertible debt owned by Darrier Hentsch.

(16)      Includes 44,272 shares of Series A preferred stock which are
convertible into 1,041,694 shares of common stock of Hybridon. This
amount also includes 238,023 shares issuable upon the exercise of
Class A warrants.

(17)      Includes 233,026 shares issuable upon the exercise of Class B warrants
held by Faisal Finance Switzerland SA.

(18)      Includes 259,375 shares issuable upon the exercise of Class C warrants
held by Finova Technology Finance Inc.

(19)      Includes 27,412 shares of Series A preferred stock which are
convertible into 644,988 shares of common stock of Hybridon. This
amount also includes 42,153 shares issuable upon the exercise of Class
A warrants, 74,265 shares issuable upon the exercise of Class D
warrants and 163,050 shares issuable upon conversion of a portion of
the $6,000,000 bank loan to Hybridon owned by this entity.

The following table sets forth certain information as of December 31,
1999, with respect to the beneficial ownership of shares of common stock and
Series A preferred stock by (i) the directors of Hybridon and (ii) the Chief
Executive Officer and other Named Executive Officers, and (iii) the directors
and executive officers of Hybridon as a group, assuming conversion of all
convertible debt or preferred stock and exercise of all warrants and stock
options by such person and only by such person.

Series A
Common Stock                Convertible Preferred Stock
------------                ---------------------------
Name of Beneficial Owner                     Amount and Nature  Percent of      Amount and Nature  Percent of
of Beneficial     Class            of Beneficial     Class
Ownership(1)                       Ownership(1)
DIRECTORS

Arthur W. Berry.......................        4,494,381(2)        21.65%           122,078(3)        18.43%
Harold W. Purkey......................        6,251,061(4)        28.23%            74,589(5)        11.26%
Youssef El-Zein.......................        2,692,339(6)        14.56%                 0                  0
Nasser Menhall........................        2,670,351(7)        14.46%                 0                  0
E. Andrews Grinstead, III ............        3,344,843(8)        17.10%                 0                  0
Sudhir Agrawal........................          619,116(9)         3.67%                 0                  0
Paul Z. Zamecnik......................          449,013(10)        2.73%                 0                  0
James B. Wyngaarden...................          123,350(11)        *                     0                  0
Camille A. Chebeir....................           25,000            *                     0                  0
H.F. Powell...........................          222,917(12)        1.35                  0                  0
All directors and executive  officers as
a group (10 persons)..................       18,315,054(13)       55.56%           196,655           29.69%
*        Less than 1%.

(1)       The number of shares beneficially owned by each director and executive
officer is determined under rules promulgated by the SEC, and the
information is not necessarily indicative of beneficial ownership for
any other purpose. Under such rules, beneficial ownership includes any
shares as to which the individual has sole or 

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shared voting power or investment power and also any shares which the
individual has the right to acquire within 60 days after December 31,
1999 through the exercise of any stock option or other right. The
inclusion herein of such shares, however, does not constitute an
admission that the named stockholder is a direct or indirect
beneficial owner of such shares. Unless otherwise indicated, each
person or entity named in the table has sole voting power and
investment power (or shares such power with his or her spouse) with
respect to all shares of capital stock listed as owned by such person
or entity.

(2)       Includes 122,078 shares of Series A preferred stock owned by four
investment advisory clients of Pecks, which clients would receive
dividends and the proceeds from the sale of such shares. Two of these
clients are Delaware State Employees Retirement Fund and Declaration
of Trust for the Defined Benefit Plan of ICI American Holdings, Inc.
These shares of Series A preferred stock are convertible into
2,872,424 shares of common stock of Hybridon. This amount also
includes 208,895 shares issuable upon the exercise of Class A warrants
and 394,354 shares issuable upon the exercise of Class D warrants held
in total by the foregoing entities. This number also includes 684,375
shares issuable upon conversion of a portion of the $6,000,000 bank
loan to Hybridon owned by certain of the foregoing entities. Mr.
Berry, a principal of Pecks, may be considered a beneficial owner of
the shares owned by such entities. Mr. Berry disclaims beneficial
ownership of these shares. This number also includes 333,333 shares
issuable upon conversion of $200,000 in convertible debt owned by Mr.
Berry.

(3)       Includes 122,078 shares of Series A preferred stock owned by four
investment advisory clients of Pecks, which clients would receive
dividends and the proceeds from the sale of such shares. Mr. Berry, a
principal of Pecks, may be considered a beneficial owner of the shares
owned by such entities. Mr. Berry disclaims beneficial ownership of
these shares.

(4)       Includes (a) 796,259 shares of common stock owned by Forum Capital
Markets LLC, (b) 328,677 shares issuable upon the exercise of Class B
warrants owned by Forum, (c) 280,517 shares issuable upon the exercise
of Class C warrants owned by Forum, (d) 468,859 shares issuable upon
the exercise of Class A warrants owned by Forum, (e) 25,812 shares
issuable upon the exercise of Class D warrants, (f) 61,568 shares
issuable upon the exercise of other warrants held by Forum, (g)
1,250,000 shares issuable upon conversion of Forum's portion of the
$6,000,000 bank loan to Hybridon, (h) 1,755,035 shares issuable upon
conversion of 74,589 shares of Series A preferred stock owned by Forum
and (i) 416,667 shares issuable upon conversion of $250,000 in
convertible debt owned by Forum. Mr. Purkey, an affiliate of Forum,
may be considered a beneficial owner of the shares beneficially owned
by such entity. This amount also includes 166,667 shares issuable upon
conversion of $100,000 in convertible debt owned by Mr. Purkey.

(5)       Consists of 74,589 shares of Series A preferred stock owned by Forum.
Mr. Purkey, an affiliate of Forum, may be considered a beneficial
owner of the shares beneficially owned by Forum.

(6)       Includes (a) 82,183 shares issuable upon the exercise of warrants held
by Mr. El-Zein, (b) 366 shares issuable upon the exercise of warrants
held by Pillar Associated, (c) 20,000 shares issuable upon the
exercise of warrants held by Pillar S.A., (d) 20,000 shares issuable
upon the exercise of warrants held by Pillar S.A.R.L., (e) 37,500
shares issuable upon the exercise of Class C warrants held by Pillar
Investment Limited, (f) 473,598 issuable upon the exercise of advisory
warrants held by Pillar Investment Limited, (g) 638,032 shares
issuable upon the exercise of placement warrants held by Pillar
Investment Limited, (h) 5,243 shares issuable upon the exercise of
other warrants held by Pillar Investment Limited, (i) 462,800 shares
held by Pillar Investment Limited, (j) 9,000 shares issuable upon the
exercise of stock options held by Mr. El-Zein, (k) 447,150 shares
issuable upon the conversion of $269,290 in convertible debt to be
issued to Pillar Investment Limited and (l) 496,833 shares issuable
upon the conversion of $298,100 in convertible debt that Pillar
Investment Limited has the right to acquire upon exercise of warrants.
Receipt by Pillar Investment Limited of the securities described in
(k) and (l) is subject to the receipt by Hybridon of a fairness
opinion. Mr. El-Zein, an affiliate of Pillar Associated, Pillar S.A.,
Pillar S.A.R.L. and Pillar Investment Limited, may be considered a
beneficial owner of the shares beneficially owned by such entities.

(7)       Includes (a) 60,195 shares issuable upon the exercise of warrants held
by Mr. Menhall, (b) 366 shares issuable upon the exercise of warrants
held by Pillar Associated, (c) 20,000 shares issuable upon the
exercise of warrants held by Pillar S.A., (d) 20,000 shares issuable
upon the exercise of warrants held by Pillar S.A.R.L., (e) 37,500
shares issuable upon the exercise of Class C warrants held by Pillar
Investment Limited, (f) 473,598 issuable

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upon the exercise of advisory warrants held by Pillar Investment
Limited, (g) 638,032 shares issuable upon the exercise of placement
warrants held by Pillar Investment Limited, (h) 5,243 shares issuable
upon the exercise of other warrants held by Pillar Investment Limited,
(i) 462,800 shares held by Pillar Investment Limited, (j) 9,000 shares
issuable upon the exercise of stock options held by Mr. Menhall, (k)
447,150 shares issuable upon the conversion of $269,290 in convertible
debt to be issued to Pillar Investment Limited and (l) 496,833 shares
issuable upon the conversion of $298,100 in convertible debt that
Pillar Investment Limited has the right to acquire upon exercise of
warrants. Receipt by Pillar Investment Limited of the securities
described in (k) and (l) is subject to the receipt by Hybridon of a
fairness opinion. Mr. Menhall, an affiliate of Pillar Associated,
Pillar S.A., Pillar S.A.R.L. and Pillar Investment Limited, may be
considered a beneficial owner of the shares beneficially owned by such
entities.

(8)       Includes 730,596 shares subject to outstanding stock options which are
exercisable within the 60-day period following December 31, 1999, as
well as 2,566,667 shares issuable upon the conversion of $1,540,000 in
convertible debt owned by Mr. Grinstead.

(9)       Includes 601,356 shares subject to outstanding stock options which are
exercisable within the 60-day period following December 31, 1999.

(10)      Includes (a) 113,250 shares subject to outstanding stock options which
are exercisable within the 60-day period following December 31, 1999,
(b) 31,250 shares issuable upon the exercise of Class C warrants and
(c) 43,333 shares issuable upon the conversion of $26,000 in
convertible debt owned by Dr. Zamecnik.

(11)      Includes (a) 118,250 shares subject to outstanding stock options which
are exercisable within the 60-day period following December 1, 1999
and (b) 700 shares held by Mr. Wyngaarden's children.

(12)      Includes 56,260 shares subject to outstanding stock options which are
exercisable within 60-day period following December 31, 1999 and
166,667 shares issuable upon the conversion of $100,000 in convertible
debt owned by Mr. Powell.

(13)      Securities owned by Pillar Associated, Pillar S.A., Pillar S.A.R.L.
and Pillar Investment Limited are included only once, although such
amounts were included above for both Messrs. El-Zein and Menhall.

CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

Since January 1, 1997, Hybridon has entered into or has been engaged in
the following transactions with the following Hybridon directors and officers,
stockholders who beneficially own more than 5% of the outstanding common stock
of Hybridon, and affiliates or immediate family members of those directors,
officers and 5% Stockholders.

TRANSACTIONS WITH PILLAR S.A. AND CERTAIN OF ITS AFFILIATES

Hybridon has entered into certain transactions with Pillar S.A., Pillar
Investment and Charles River Building Limited Partnership, the entity which
owned Hybridon's former headquarters in Cambridge, Massachusetts (the "Cambridge
Landlord"). Pillar S.A. and Pillar Investment are affiliates of Messrs. El-Zein
and Menhall, two directors of Hybridon. The Cambridge Landlord is an affiliate
of Messrs. El-Zein and Menhall and Mohamed El-Khereiji, a former director of
Hybridon. The following is a summary of those transactions that relate to
Hybridon's 1998 fiscal year.

In 1997 and 1998, Hybridon was a party to a consulting agreement with
Pillar S.A. dated as of March 1, 1994, under which Pillar S.A. provided Hybridon
with financial advisory and managerial services in connection with Hybridon's
overseas operations, including support services in connection with contracts and
agreements. Under the terms of the 1994 Pillar consulting agreement, Hybridon
paid Pillar S.A. consulting fees of $60,000 per month and $23,000 per month for
overhead costs, and reimbursed certain authorized out-of-pocket expenses. The
1994 Pillar consulting agreement expired on February 28, 1998. Pursuant to the
1994 Pillar consulting agreement, Hybridon issued to Pillar S.A two five year
warrants to purchase an aggregate of 40,000 shares of Hybridon common stock.

On July 8, 1995, Hybridon entered into an additional agreement with
Pillar S.A. pursuant to which Pillar S.A. agreed for a period of two years to
provide to Hybridon certain consulting, advisory and related services, in
addition to

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the services to be provided under the 1994 Pillar Consulting Agreement, and
serve as Hybridon's exclusive agent in connection with potential corporate
partnerships in Europe and as a non-exclusive placement agent of Hybridon in
connection with private placements of securities of Hybridon. On November 1,
1995, the Pillar Europe agreement was amended to provide that (1) Pillar S.A.
would cease to serve as Hybridon's executive agent in connection with potential
corporate partnerships in Europe, but would continue to serve as a non-exclusive
agent in that connection, (2) Pillar S.A. would receive a retainer of $26,470
per month for the balance of the term of the Pillar Europe agreement, (3) the
fees provided for in the Pillar Europe agreement would only be payable to Pillar
S.A. in connection with potential collaborations with any French pharmaceutical
company with which Hybridon was involved in discussions during the 12-month
period ended November 1, 1995 as a result of introductions by Pillar S.A., and
(4) any compensation payable to Pillar S.A. in connection with its services with
respect to other corporate collaborations or any placements of securities would
be negotiated on a case-by-case basis and would be subject to the approval of
the independent members of the board of directors of Hybridon. The Pillar Europe
agreement expired on April 1, 1997.

In 1998, Hybridon paid Pillar Investment a total of $300,000 under
these agreements, in the form of 150,000 shares of common stock and warrants to
purchase 37,500 shares of common stock, at an exercise price of $2.40 per share,
subject to adjustment, in lieu of cash. In 1997, Hybridon paid Pillar S.A.
$903,267 under the 1994 Pillar consulting agreement and the Pillar Europe
agreement.

Hybridon has retained Pillar Investment as placement agent in
connection with the private placements of securities of Hybridon in offshore
transactions in reliance upon an exemption from registration under Regulation S
promulgated under the Securities Act of 1933. Pillar Investment received fees
consisting of (1) 9% of the gross proceeds of each Regulation S Offering, (2) a
non-accountable expense allowance equal to 4% of those gross proceeds, (3) the
right to purchase, for nominal consideration, warrants to purchase 473,598
shares of common stock, at an exercise price of $2.40 per share, subject to
adjustment, (4) the right to purchase, for nominal consideration, warrants to
purchase a number of shares of the common stock of Hybridon equal to 10% of the
total number of shares of common stock sold by Hybridon for which Pillar
Investment acted as placement agent, exercisable at 120% of the relevant common
stock offering price, for a period of five years, resulting, as of the date
hereof, in the right to receive warrants to purchase 638,032 shares at $2.40 per
share, subject to adjustment, and (5) a consulting/restructuring fee of $960,000
payable in common stock of Hybridon valued at the market price and payable in
three equal installments as net proceeds of $25,000,000, $30,000,000 and
$35,000,000 are received in the aggregate from private placements effected by
Hybridon in 1998 to the extent contemplated by the consent and waiver dated as
of January 12, 1998 given by certain beneficial holders of Hybridon's 9%
convertible subordinated notes, or otherwise to the extent contemplated by the
Placement Agency agreement between Hybridon and Pillar Investment, subject to
Hybridon's receiving of a fairness opinion regarding this. Pillar Investment may
not receive compensation in excess of the level that was approved by the holders
of the 9% notes. Pillar Investment has received $1,635,400 in cash pursuant to
these arrangements and Pillar has received warrants to purchase 1,111,630 shares
of common stock.

In addition, in connection with the Regulation S offerings, Hybridon
and Pillar Investment have entered into an advisory agreement dated May 5, 1998,
under which Pillar Investment acts as Hybridon's non-exclusive financial
advisor. This agreement requires that Hybridon pay an affiliate of Pillar
Investment a monthly retainer of $5,000, with a minimum engagement of 24 months
beginning on May 5, 1998, and further provides that Pillar Investment is
entitled to receive (1) out-of-pocket expenses, (2) subject to Hybridon's
receiving a fairness opinion on this matter, 300,000 shares of common stock in
connection with Pillar Investment's efforts in assisting Hybridon in
restructuring its balance sheet, and (3) certain cash and equity success fees in
the event Pillar Investment assists Hybridon in connection with certain
financial and strategic transactions. As of April 16, 1999, Hybridon issued to
Pillar Investment the stipulated 300,000 shares of common stock. Hybridon
received a fairness opinion in connection with that issuance. In addition,
Hybridon was a party to a lease with a third party dated March 23, 1994 for
approximately 1,800 square feet of space in Paris, France. Hybridon's
obligations under the Paris lease was guaranteed by Pillar S.A. Hybridon
terminated the Paris lease on March 31, 1998. Pursuant to a 1999 private
placement offering, Hybridon sold 8% notes to certain investors, including some
investors that Pillar Investment introduced to Hybridon. In connection with this
offering, and in lieu of any compensation due under the financial advisory
agreement between Hybridon and Pillar Investment, Hybridon agreed to pay Pillar
Investment's reasonable expenses and to issue to Pillar Investment and its
designees additional 8% notes in an aggregate principal amount equal to 9% of
the aggregate principal amount of 8% notes purchased by those Pillar-introduced
investors. Hybridon also agreed to issue to Pillar Investment and its designees
warrants to purchase additional 8% notes in an aggregate principal amount equal
to 10% of the aggregate principal amount of 8% notes purchased by those
Pillar-introduced investors. These warrants have a strike price equal to 110% of
the principal amount of the 8% notes purchasable thereunder. Hybridon's
obligations to issue the 8% notes and the warrants and to reimburse Pillar
Investment's 

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expenses are subject to the condition precedent that Hybridon will have had
delivered to it a fairness opinion in form and substance deemed by Hybridon, in
its sole discretion, to satisfy the requirements of the indenture relating to
Hybridon's 9% notes. As of December 31, 1999, Pillar Investment had earned the
right to receive $269,290 in 8% notes and warrants to purchase an additional
$298,100 in 8% notes.

TRANSACTIONS WITH THE CAMBRIDGE LANDLORD

From February 4, 1997 to September 16, 1998, Hybridon was a party to a
lease with the Cambridge Landlord for its Cambridge facilities. The Cambridge
Lease originally provided for an annual rent equal to $30 per square foot on a
triple-net basis, where the tenant pays taxes, insurance, and operating costs,
for the first five years, $33 per square foot on a triple-net basis for the next
five years and the greater of $30 per square foot on a triple-net basis or the
then-market value of leased property for each of the five-year renewal terms. In
connection with Hybridon's election to acquire an interest in the Cambridge
Landlord, as described below, the annual rent due under the Cambridge lease was
increased for the first five years of the lease term to $38 per square foot on a
triple-net basis, for the second five years to $42 per square foot on a
triple-net basis and for the third five years to $47 per square foot on a
triple-net basis.

On July 1, 1996, Hybridon decided to fund approximately $5.5 million of
the costs, primarily relating to tenant improvements, of the construction of the
leased premises through contributions to the capital of the Cambridge Landlord
in exchange for a limited partnership interest in the Cambridge Landlord. The
partnership interest entitled Hybridon to an approximately 32% interest in the
Cambridge Landlord. Hybridon had the right, for a period of three years ending
February 2000, to sell the partnership interest back to certain limited partners
of the Cambridge Landlord for a price equal to the greater of (1) the total cash
contribution made by Hybridon to the Cambridge Landlord or (2) the fair market
value of the partnership interest at the time.

In 1997, Hybridon had on deposit with Bank fur Vermogensanlagen und
Handel the amount of $1,034,618. In November 1997, German banking authorities
imposed a moratorium on Bank fur Vermogensanlagen und Handel and closed Bank fur
Vermogensanlagen und Handel for business. Pursuant to an agreement dated
November 28, 1997, the Cambridge Landlord agreed to assume the risk for the Bank
fur Vermogensanlagen und Handel deposit and to pay to Hybridon the amount of
$75,000 a month after each rent payment under the Cambridge lease was made until
such time as $1,000,000 had been paid to Hybridon or the Bank fur
Vermogensanlagen und Handel deposit was released.

In June 1998, Hybridon moved its headquarters from the Cambridge
facility to its facility in Milford, Massachusetts. The Cambridge facility was
re-leased in September 1998 to a third party, subject to a sublease of a portion
of the facility. As a result, Hybridon terminated the Cambridge lease and was
relieved of its substantial lease obligations under the Cambridge lease, subject
to a contingent continuing liability for any sublessee defaults. Further, in
November 1998 Hybridon completed the sale of its partnership interest. As a
result of these transactions, Hybridon received $6,163,000 from the Cambridge
Landlord, which included payment for the partnership interest, the return of a
portion of the security deposit required under the Cambridge lease, and payment
in full of the Bank fur Vermogensanlagen und Handel deposit. Hybridon has agreed
to reimburse the Cambridge landlord for any cash received under this agreement,
up to the amount realized by Hybridon from the final settlement of the Bank fur
Vermogensanlagen und Handel deposit, after the moratorium is lifted.

TRANSACTIONS WITH FORUM CAPITAL MARKETS LLC AND PECKS MANAGEMENT PARTNERS LTD.

In 1998, Hybridon entered into certain transactions with Forum, an
affiliate of Mr. Purkey, a director of Hybridon, and entities advised by Pecks
Management Partners Ltd. Mr. Berry, a principal of Pecks, is a director of
Hybridon.

Hybridon retained Forum as a placement agent of Hybridon in connection
with Hybridon's 1998 Regulation D offering of Series A preferred stock and Class
D warrants in the U.S. Forum received as compensation for its services as
placement agent with regard to the Regulation D offering and its assistance with
an exchange offer made by Hybridon to the holders of its 9% notes, 597,699
shares of common stock and warrants to purchase prior to May 4, 2003 a total of
609,194 shares of common stock exercisable at $2.40 per share, in each case
subject to adjustment. In addition, in exchange of the agreements made by Forum
consenting to the Regulation D offering and waiving certain obligations of
Hybridon to Forum, Hybridon agreed to amend Forum's warrant dated as of April 2,
1997, to purchase up to 71,301 shares of common stock of Hybridon, to change the
exercise price to $4.25 per share, subject to adjustment, 

-47-

and increase the number of shares of common stock purchasable upon exercise to
588,235, in each case subject to adjustment, and to provide that it may not be
exercised until May 5, 1999 and the transactions contemplated by those private
placements and by the exchange offer will not trigger any anti-dilution
adjustments to its exercise price or the number of shares of common stock
purchasable upon exercise.

In November 1998, Forum and entities advised by Pecks purchased
Hybridon's bank loan. In connection with the purchase of the loan, the
purchasing entities advanced an additional amount to Hybridon so as to increase
the outstanding principal amount of the loan to $6,000,000. In addition, the
purchasing entities agreed to amend the terms of the loan. This principal amount
of the loan and unpaid interest thereon is convertible, in whole or in part, at
the lenders' option into common stock at a conversion price of $2.40 per share.

In connection with the purchase of the loan, Forum received a fee of
$400,000, which Forum has reinvested by purchasing from Hybridon 160,000 shares
of common stock and warrants to purchase an additional 40,000 shares of common
stock at $3.00 per share. In addition, Forum received warrants exercisable until
maturity of the Loan to purchase 133,333 shares of common stock at $3.00 per
share.

In connection with the offering of these notes, Forum and the entities
advised by Pecks entered into a Subordination and Intercreditor Agreement with
Hybridon and the representative of the purchasers of the notes whereby, among
other things, they agreed to subordinate their loan to the notes, subject to
certain conditions. Also in connection with this offering, Hybridon agreed to
issue warrants to purchase an aggregate of 2.75 million shares of Hybridon's
common stock to designees of Pecks and Forum. These warrants are exercisable
from December 31, 2000 until December 31, 2002 at $0.60 per share.

Hybridon maintains an investment account at Forest Investment
Management LLC, an affiliate of Forum and Mr. Purkey.

OTHER TRANSACTIONS

In March 1999, Hybridon entered into consulting arrangements with each
of Mr. Powell, Dr. Zamecnik and Dr. Wyngaarden providing that each of them will
act as a consultant to Hybridon for a two-year period and will receive a
consulting fee of $20,000 per year for general consulting services. In addition,
each agreement provides that they each will receive a consulting fee of $1,500
per day of on-site consulting services they provide at Hybridon's corporate
offices, or at an alternative site agreed upon by the parties, and at Hybridon's
prior request. Additional fees for special projects will be negotiated
separately between the parties. Each of Mr. Powell, Dr. Zamecnik and Dr.
Wyngaarden also received options to purchase 150,000 shares of Hybridon's common
stock at $2.00 per share; such options will vest over a two-year period. Dr.
Zamecnik has received $26,000 in convertible notes for his 1999 consulting
services and board fees, which he may at his option convert into 43,333 shares
of common stock. Mr. Powell's consulting agreement terminated when Mr. Powell
resigned from the board of directors of Hybridon in February 2000.

Certain persons and entities, including Dr. Zamecnik, Pillar S.A.,
Pillar Limited, Forum, the entities advised by Pecks, Intercity Holdings, Mr.
Bin Laden and Nicris Limited, are entitled to certain rights with respect to the
registration under the Securities Act of certain shares of Hybridon's common
stock, including shares of common stock that may be acquired pursuant to the
exercise of options or warrants, under the terms of agreements among Hybridon
and the rightsholders. The registration agreements generally provide that in the
event Hybridon proposes to register any of its securities under the Securities
Act at any time, with certain exceptions, the rightsholders, including Pillar
S.A., Pillar Limited, Intercity Holdings, Mr. Bin Laden and Nicris Limited, but
excluding, among others, Dr. Zamecnik, have the additional right under certain
registration agreements to require Hybridon to prepare and file registration
statements under the Securities Act, if rightsholders holding specified
percentages of the registrable shares so request, and Hybridon is required to
use its best efforts to effect that registration, subject to certain conditions
and limitations.

Hybridon sold an aggregate of $1,500,000 principal amount of promissory
notes to E. Andrews Grinstead, III, Hybridon's Chief Executive Officer, at face
value during September and November of 1999. These notes accrued interest at 12%
per annum (15% upon Hybridon's election to pay this interest in shares of common
stock rather than cash) and, upon the closing of any third-party debt financing
that closed on or before March 1,

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2000, were intended to be converted into the debt sold in that financing. These
notes have, together with $40,000 in accrued interest, been converted into 8%
notes of Hybridon due 2002.

In addition , in connection with the financing conducted in December
1999, other Hybridon directors and certain affiliates of Hybridon directors
purchased Hybridon 8% notes in the amount set forth below:

Forum Capital Markets LLC           $250,000  
Arthur W. Berry                     $100,000  
Harold w. Purkey                    $100,000  
H. F. Powell                        $100,000  

Two other principals of Forum Capital Markest LLC each purchased
$100,000 of the 8% notes.

Hybridon believes that the terms of the transactions described above
were no less favorable than Hybridon could have obtained from unaffiliated third
parties.

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SELLING STOCKHOLDERS

The tables below set forth, to the knowledge of Hybridon, certain
information as of December 31, 1999 with respect to the selling stockholders.
The table entitled "Stockholders Selling Common Stock" includes information with
respect to selling stockholders who are selling common stock in this offering.
The table entitled "Stockholders Selling Preferred Stock" includes information
with respect to selling stock holders who are selling preferred stock in this
offering. Except as noted below, no selling stockholder selling common or
preferred stock in this offering will beneficially own 1% or more of the
outstanding stock of Hybridon after the offering.

Except as described below, none of the selling stockholders holds any
position or office with, or has otherwise had a material relationship with,
Hybridon within the past three years.

Stockholders Selling Common Stock

Number of Shares of                                Number of Shares of
Common Stock Beneficially    Number of Shares of          Common Stock
Owned                     Common             Beneficially Owned
Name of Selling Stockholder            Prior to Offering1     Stock Included in Offering    After Offering1
---------------------------            ------------------     --------------------------    ---------------

Fouad  M.O.  Tawfig  and  Hanan H.            330,876                    6,250                 324,626
Zagzoug
Torben Duer                                   126,750                   18,750                 108,000
Thomas Fr. Duer                                62,500                   62,500                       0
Darier Hentsch & Cie                        1,317,755                  651,088                 666,667
Finn Trunk Black                                3,750                    3,750                       0
MM Pictet & Cie                               588,000                  588,000                       0
Nicris Limited7                             1,360,644                1,050,644                 310,000
Raji Abou Hadar                               395,833                   62,500                 333,333
Intercity Ltd.7                             2,216,666                1,875,000                 341,666
Clapham Investments Ltd.                      458,833                  125,000                 333,833
LGT Bank in Liechtenstein AG                  312,500                  312,500                       0
Participations Besancon                       125,000                  125,000                       0
Loxhall Limited                                62,500                   62,500                       0
MicroTech Software a/s                         33,000                   31,250                  11,750
JSP Holdings ApS                               24,500                   12,500                  12,000
Jan Poulson                                    18,750                   18,750                       0
Mr. Mohamad Hassan Abdul Ghani                 67,717                   67,717                       0
Dr. Khaled M.R. Abdul Ghani                   635,435                  135,435                 500,000
Mr. Imad Mustapha Mansour                      67,717                   67,717                       0
Mr. Malek Salam                                88,033                   88,033                       0
Faisal Finance (Switzerland) S.A.           1,043,113                1,009,779                  33,334
Mr. Guy Semon                                  22,149                   22,149                       0
Mrs. Francoise Semon                           22,149                   22,149                       0
Mr. Le Pelley Dumanoir                         22,149                   22,149                       0
Mr. Moh'd Abdo Sweidan                         67,119                   67,119                       0
Mr. Isam Moh'd Khairy Kabbani                  67,119                   67,119                       0
Dr. Essam Ahmad Jawadm Alamdar                301,357                  201,357                 100,000
Arab Islamic Bank (E.C.)                      503,394                  503,394                       0
Mr. Sobbi Adra                                 23,492                   23,492                       0
Mr. Mansour S.M.A. Al-Sharif                  107,639                   65,972                  41,667
Mr. Nafez M.M. Al-Jindi                        65,972                   65,972                       0
Solter Corporation                            467,345                  196,047                 271,298
Carset Overseas Corporation                   176,375                  176,375                       0
Mr. Ali A. Bajrai                             163,310                  163,310                       0
Pillar Investment Limited2                  2,560,356                1,599,130                 961,226
Bioreliance Corporation                        16,697                   16,697                       0
Chestnut Partners                              62,500                   62,500                       0
Datamonitor                                    62,500                   62,500                       0
Finova Technology Finance, Inc.               896,875                  896,875                       0
HPC America, Inc.                             218,750                  218,750                       0
Hyal Pharmaceutical Corporation                17,500                   17,500                       0
SEIF Foundation                               319,725                  119,725                 200,000
Janitronics                                    45,724                   45,724                       0
Kinetic Systems, Inc.                         163,238                  163,238                       0
Massachusetts Eye & Ear Infirmary              62,500                   62,500                       0
Norwegian Radium Hospital                      37,500                   37,500                       0
Research Foundation
Susan and Anthony Russo                        62,500                   62,500                       0
Pharmakinetics Laboratories, Inc.              55,803                   55,803                       0

-50-

Number of Shares of                                Number of Shares of
Common Stock Beneficially    Number of Shares of          Common Stock
Owned                     Common             Beneficially Owned
Name of Selling Stockholder            Prior to Offering1     Stock Included in Offering    After Offering1
---------------------------            ------------------     --------------------------    ---------------

The Perkin Elmer Corporation                  205,377                  205,377                       0
Primedica Corporation                         364,418                  364,418                       0
Quintiles Transnational Corp.                 379,175                  379,175                       0
Siena Construction Corporation                 31,250                   31,250                       0
Sierra Biomedical, Inc.                       150,203                  150,203                       0
SP Pharmaceuticals LLC                        115,985                  115,985                       0
Southern Research Institute                    68,860                   68,860                       0
Transamerica Business Credit                  318,750                  318,750                       0
Corporation
Triumvirate Environmental, Inc.                19,138                   19,138                       0
University of Kansas                           29,260                   29,260                       0
University of Massachusetts                    84,450                   84,450                       0
Paul C. Zamecnik and Mary V.                  449,013                  156,250                 292,763
Zamecnik, JTWROS3,7
Allstate Insurance Company                    499,895                  499,895                       0
Angelo Gordon & Co., L.P.                     116,636                  116,636                       0
Michael Angelo, L.P.                          316,627                  316,627                       0
Ramius Fund Ltd.                              233,272                  233,272                       0
Raphael, L.P.                                 316,627                  316,627                       0
Medici Partners, L.P.                          99,961                   99,961                       0
CNA Income Shares, Inc.                       499,895                  499,895                       0
Forest Alternative Strategies                  26,653                   26,653                       0
Fund II, L.P. Series A5I4
Forest Alternative Strategies                  13,350                   13,350                       0
Fund II, L.P. Series A5M4
Forest Alternative Strategies                     744                      744                       0
Fund II, L.P. Series B-34
Forest Fulcrum Ltd.4                          108,310                  108,310                       0
Forest Global Convertible Fund                160,441                  160,441                       0
Series A54
Forest Greyhound4                               6,199                    6,199                       0
Forest Performance Fund4                        7,152                    7,152                       0
LLT Ltd. 4                                     26,653                   26,653                       0
Forest Convertible Fund                        17,106                   17,106                       0
Forum Capital Markets LLC5,7                6,083,394                4,378,167               1,705,227
Providian Life & Health                       672,204                  672,204                       0
Monumental Life Insurance Co.                 546,769                  546,769                       0
The Guardian Pension Trust Fund                99,961                   99,961                       0
Harris Investment Management                   92,524                   92,524                       0
Offshore Strategies Ltd.                      333,307                  333,307                       0
Libertyview Plus Fund                          49,007                   49,007                       0
Libertyview Fund LLC                           24,507                   24,507                       0
CPR (USA)                                     113,623                  113,623                       0
Lincoln National Life Insurance             1,279,717                1,279,717                       0
Co.
Lincoln National Convertible                  496,594                  496,594                       0
Securities  Fund
Weirton Trust                                 144,989                  144,989                       0
Walker Art Center                              10,230                   10,230                       0
United National Insurance Co.                  23,330                   23,330                       0
Equi Select Growth & Income Fund              166,642                  166,642                       0
Zazove Convertible Fund, L.P.                 159,530                  159,530                       0
Lois Wilkens                                    6,389                    6,389                       0
Winchester Convertible Plus Ltd.              129,988                  129,988                       0
Foundation Account  No. 1                      69,983                   69,983                       0
LLC Account No. 1                              33,328                   33,328                       0
GPS Fund Limited                               99,959                   99,959                       0
Telefix (First Delta)                          16,676                   16,676                       0
Guardian Life Insurance Co. of              3,255,110                3,255,110                       0
America
Declaration of Trust for the                  924,456                  741,406                    163,050
Defined Benefits Plan of ICI
America Holdings, Inc.7
J.W. McConnell Family Foundation6              65,988                    8,988                     57,000
Delaware State Employees                    2,552,933                2,194,683                    355,250
Retirement Fund6,7
General Motors Employees Domestic           3,832,220                3,266,595                    565,625
Group Trust7

-51-

Number of Shares of                                Number of Shares of
Common Stock Beneficially    Number of Shares of          Common Stock
Owned                     Common             Beneficially Owned
Name of Selling Stockholder            Prior to Offering1     Stock Included in Offering    After Offering1
---------------------------            ------------------     --------------------------    ---------------

Zeneca Holdings6                              619,670                  510,595                    109,075
Thermo Electron Balanced                        8,871                    8,871                          0
Investment Fund
Tucker Anthony & R.L. Day, Inc.                 6,199                    6,199                          0

NOTES:

1.       Includes common stock issuable upon the exercise of stock options,
warrants, convertible preferred stock and convertible debt.

2.       Mr. Nasser Menhall and Mr. Youssef El-Zein, members of the board of
directors of Hybridon, are principals of Pillar Investment Limited.

3.       Dr. Zamecnik is a member of the board of directors of Hybridon and is a
consultant to Hybridon.

4.       Harold W. Purkey, a member of the board of directors of Hybridon, is an
affiliate of this selling stockholder.

5.       Harold W. Purkey, a member of the board of directors of Hybridon, is
the President and a 10% owner of Forum Capital Markets.

6.       Arthur W. Berry, a member of the board of directors of Hybridon, serves
as investment advisor to this selling stockholder.

7.       These selling stockholders will beneficially own greater than 1% of
Hybridon's common stock (which for purposes of this calculation
includes common stock issuable upon exercise of warrants or conversion
of convertible debt within 60 days after December 31, 1999) after the
offering, as follows:

Percentage of Outstanding Common
Selling Stockholder                                  Stock Beneficially Owned After the Offering
-------------------                                  -------------------------------------------

Forum Capital Markets LLC                                  9.51%
Pillar Investment Limited                                  5.79%
Darrier Hentsch & Cie                                      4.73%
General Motors Employees Domestic Group Trust              3.36%
Delaware State Employees Retirement Fund                   2.14%
Intercity Ltd.                                             2.10%
Raji Abou Hader                                            2.01%
Clapham Investments Ltd.                                   2.01%
Fouad M.O. Tawfig and Hanan H. Zagzoug                     1.96%
Nicris Limited                                             1.91%
Solter Corporation                                         1.64%
Khaled M.R. Abdul Ghani                                    1.21%
Paul C. Zamecnik and Mary V. Zamecnik, JTWROS              1.13%

Stockholders Selling Preferred Stock

Number of Shares of                                           
Convertible      Number of Shares of  Number of Shares of 
Preferred           Convertible          Convertible     
Stock Beneficially        Preferred            Preferred      
Name of                 Owned Prior to      Stock Included in   Stock Beneficially  
Selling Stockholder              Offering             Offering        Owned After Offering
-------------------              --------             --------        --------------------

Allstate Insurance Company               17,294               17,294                  0
Angelo Gordon & Co., L.P.                 4,035                4,035                  0
Michael Angelo, L.P.                     10,954               10,954                  0
Ramius Fund Ltd.                          8,070                8,070                  0
Raphael, L.P.                            10,954               10,954                  0
Medici Partners, L.P.                     3,458                3,458                  0
CNA Income Shares, Inc.                  17,294               17,294                  0
Forest  Alternative   Strategies            922                  922                  0
Fund II, L.P. Series A5I1
Forest  Alternative   Strategies            462                  462                  0
Fund II, L.P. Series A5M1
Forest Fulcrum Ltd.1                      3,747                3,747                  0
Forest Global  Convertible  Fund          5,765                5,765                  0
Series A51
Forest Performance Fund1                    138                  138                  0
Forest Convertible Fund1                    727                  727                  0
LLT Ltd.1                                   922                  922                  0
Forum Capital Markets LLC2               74,589               74,589                  0

-52-

Number of Shares of                                           
Convertible      Number of Shares of  Number of Shares of 
Preferred           Convertible          Convertible     
Stock Beneficially        Preferred            Preferred      
Name of                 Owned Prior to      Stock Included in   Stock Beneficially  
Selling Stockholder              Offering             Offering        Owned After Offering
-------------------              --------             --------        --------------------

Providian Life & Health                  22,264               22,264                  0
Monumental Life Insurance Co.            16,933               16,933                  0
The Guardian Pension Trust Fund           3,458                3,458                  0
Harris Investment Management              3,201                3,201                  0
Offshore Strategies Ltd.                 11,531               11,531                  0
Libertyview Plus Fund                       497                  497                  0
Libertyview Fund LLC                        248                  248                  0
CPR (USA)                                   864                  864                  0
Lincoln National Life Insurance          44,272               44,272                  0
Co.
Lincoln National Convertible             17,180               17,180                  0
Securities Fund
Weirton Trust                             5,016                5,016                  0
United National Insurance Co.               807                  807                  0
Equi Select Growth & Income Fund          5,765                5,765                  0
Zazove Convertible Fund, L.P.             5,519                5,519                  0
Lois Wilkens                                221                  221                  0
Winchester Convertible Plus Ltd.          4,497                4,497                  0
Foundation Account  No. 1                 2,421                2,421                  0
LLC Account No. 1                         1,153                1,153                  0
GPS Fund Limited                          3,458                3,458                  0
Telefix (First Delta)                       577                  577                  0
Guardian Life Insurance Co. of          112,612              112,612                  0
America
Declaration of Trust for the             27,412               27,412                  0
Defined Benefits Plan of ICI
America Holdings, Inc.3
J.W. McConnell Family Foundation            382                  382                  0
Delaware State Employees                 75,926               75,926                  0
Retirement Fund3
General Motors Employees                117,887              117,887                  0
Domestic Group Trust
Zeneca Holdings3                         18,358               18,358                  0
Thermo Electron Balanced                    377                  377                  0
Investment Fund

--------------------------------- -------------------- -------------------- --------------------

NOTES:

1.       Harold W. Purkey, a member of the board of directors of Hybridon, is an
affiliate of this selling stockholder.

2.       Harold W. Purkey, a member of the board of directors of Hybridon, is
the President and a 10% owner of Forum Capital Markets.

3.       Arthur W. Berry, a member of the board of directors of Hybridon, serves
as investment advisor to this selling stockholder.

-53-

DESCRIPTION OF CAPITAL STOCK

The authorized capital stock of Hybridon consists of 100,000,000 shares
of common stock and 5,000,000 shares of preferred stock, par value $.01 per
share, of which 1,500,000 have been designated as convertible preferred stock.
On January 31, 2000, there were issued and outstanding 16,262,722 shares of
common stock and 662,167 shares of convertible preferred stock.

There follows a brief summary of the terms of the common stock and the
convertible preferred stock. For further information please refer to the
restated certificate of incorporation of Hybridon, including the certificate of
designation for the Series A preferred stock, which is filed as an exhibit to
the registration statement.

COMMON STOCK

Holders of common stock are entitled to one vote for each share held on
all matters submitted to a vote of stockholders and do not have cumulative
voting rights. Accordingly, holders of a majority of the shares of common stock
entitled to vote in any election of directors may elect all of the directors
standing for election. Holders of common stock are entitled to receive ratably
such any dividends declared by the board of directors out of legally available
funds, subject to any preferential dividend rights of the preferred stock or
other securities. Upon the liquidation, dissolution or winding up of Hybridon,
the holders of common stock are entitled to receive ratably the net assets of
Hybridon available after the payment of all debts and other liabilities and
subject to the prior rights of any outstanding shares of preferred stock and to
the Liquidation Put Right described in the next paragraph. Holders of common
stock have no preemptive, subscription, redemption or conversion rights. The
rights, preferences and privileges of holders of common stock are subject to,
and may be adversely affected by, the rights of the holders of shares of any
series of preferred stock that Hybridon may designate and issue in the future,
and the rights of creditors of Hybridon.

Pursuant to the terms of the Unit Purchase Agreement, the initial
purchasers of certain of the shares of common stock sold in the Regulation S and
the Regulation D offerings (those shares, the "Put Shares" those purchasers, the
"Liquidation Put Holders") have the right to put those shares back to Hybridon
upon the liquidation of Hybridon, but only after all other indebtedness and
obligations of Hybridon and all rights of any holders of any capital stock
ranking prior and senior to the common stock with respect to liquidation have
been satisfied in full (that right, the "Liquidation Put"). The Liquidation Put
is not transferable, and therefore purchasers of common stock pursuant to this
prospectus will not be able to exercise the Liquidation Put with respect to
those shares. Any Liquidation Put Holders that have not sold or otherwise
transferred any Put Shares will, however, be able to exercise the Liquidation
Put with respect to those Put Shares upon a liquidation of Hybridon.
Consequently, in the event of liquidation of Hybridon, holders of shares of
common stock that are not subject to the Liquidation Put right may receive
smaller liquidation distributions per share than they would have had no
Liquidation Put Holders exercised the Liquidation Put. As of January 31, 2000,
there were 9,246,476 Put Shares outstanding.

PREFERRED STOCK

Under the terms of the restated certificate of incorporation, the board
of directors is authorized, subject to any limitations prescribed by law,
without stockholder approval, to issue up to 5,000,000 shares of preferred stock
in one or more series with such rights, preferences, privileges and
restrictions, including voting rights, dividend rights, conversion rights,
redemption privileges and liquidation preferences, as the board of directors
determines.

SERIES A PREFERRED STOCK

Dividends. Each share of Series A preferred stock is entitled to
receive cumulative semi-annual dividends payable, at the option of Hybridon, in
cash or additional shares of convertible preferred stock, at the rate of 6.5%
per annum plus accrued but unpaid dividends. Dividends accrue from the date of
issuance and paid semi-annually on April 1 and October 1 of each year or, if any
such day is not a business day, on the next business day. Dividends are paid, at
the election of Hybridon, either in cash or additional shares of convertible
preferred stock. In calculating the number of shares of convertible preferred
stock to be paid with respect to each dividend, the convertible preferred stock
is valued at $100.00 per share (subject to appropriate adjustment to reflect any
stock split, combination, reclassification or reorganization of the convertible
preferred stock).

-54-

Liquidation Preference. In the event of a (1) liquidation, dissolution
or winding up of Hybridon, whether voluntary or involuntary, (2) a sale or other
disposition of all or substantially all of the assets of Hybridon, or (3) any
consolidation, merger, combination, reorganization or other transaction in which
Hybridon is not the surviving entity or if stock constituting more than 50% of
Hybridon's voting power is exchanged for or changed into stock or securities of
another entity, cash, or any other property (a "Merger Transaction") (items (1),
(2) and (3) of this sentence being collectively referred to as a "Liquidation
Event"), after payment of debts and other liabilities of Hybridon, the holders
of shares of convertible preferred stock will be entitled to be paid out of
Hybridon's available assets, before any payment to holders of shares ranking
junior to the convertible preferred stock, an amount equal to the Dividend Base
Amount. In the case of a Merger Transaction, however, this payment may be made
in cash, property or securities of the entity surviving the Merger Transaction.
If upon any Liquidation Event, whether voluntary or involuntary, the assets to
be distributed to the holders of the convertible preferred stock are
insufficient to permit the payment to such shareholders of the full amount owed,
then all of Hybridon's available assets will be distributed ratably to the
holders of the convertible preferred stock. All shares of convertible preferred
stock rank, as to payment upon the occurrence of any Liquidation Event, senior
to the common stock and senior to all other series of preferred stock, unless
the terms of any Series provides otherwise.

Right of Conversion. Commencing after May 5, 1999, shares of
convertible preferred stock became convertible, at the option of the holder,
into shares of common stock or other securities and property. The initial
conversion price per share of common stock (the "Conversion Price") is $4.25,
and is subject to adjustment as described below. The rate at which each share of
convertible preferred stock is convertible at any time into common stock (the
"Conversion Rate") will be determined by dividing the then-existing Conversion
Price into the "Dividend Base Amount" of a share of convertible preferred stock,
which is equal to $100 plus accrued but unpaid dividends (subject to adjustment
to reflect any stock split, combination, reclassification or reorganization of
the convertible preferred stock).

Adjustment of Conversion Rate and Conversion Price. As of June 15,
1999, each share of convertible preferred stock was convertible into
approximately 23.53 shares of common stock. In order to preserve the economic
value of shares of convertible preferred stock, the Conversion Price will be
adjusted if Hybridon does the following;

o        pays a dividend or makes a distribution on any class of capital stock
in shares of its common stock;

o        subdivides its outstanding common stock into a greater number of
shares;

o        combines its outstanding common stock into a smaller number of shares;

o        issues shares of common stock or preferred stock to any holder of
common stock or preferred stock rights to acquire shares of common
stock or preferred stock at a price per share less than the market
price (as defined);

o        pays or distributes to the holders of common stock or preferred stock
assets, properties, or rights to acquire Hybridon Capital Stock at a
price per share less than the market price; or

o        makes a distribution consistently solely of cash to the holders of any
class of capital stock where, during a specified 12-month period, the
cash distribution exceeds 10% of the product of the market price of the
common stock multiplied by the total outstanding common stock.

Exceptions to Adjustments. No adjustment will, however, be made to
either the Conversion Rate or the Conversion Price for issuances of common stock
or preferred stock or cash paid to holders of shares of convertible preferred
stock (1) as payment for accrued dividends or (2) as a mandatory conversion or
mandatory redemption payment as described below.

Other Changes in Conversion Rate. Hybridon from time to time may
increase the Conversion Rate by any amount for any period of time if the period
is at least 20 days and if the increase is irrevocable during the period.
Whenever the Conversion Rate is so increased, Hybridon will notify registered
holders.

Hybridon may also increase the Conversion Rate in order to avoid or
diminish any income tax to holders of common stock resulting from any dividend
or distribution of stock or issuance of rights or warrants to purchase or
subscribe for stock or from any event treated as such for income tax purposes.

-55-

The Conversion Price may not be adjusted to an amount less than $.001
per share, the current par value of the common stock into which the convertible
preferred stock is convertible.

Mandatory Conversion and Redemption. Upon giving notice to the holders
of the convertible preferred stock, Hybridon may, at its option, cause the
convertible preferred stock to be converted in whole or in part, on a pro rata
basis, into shares of common stock using a Conversion Price equal to $4.00 if
the closing bid price of the common stock equals or exceeds 250% of the
Conversion Price for at least 20 trading days in any period of 30 consecutive
trading days.

At any time after April 1, 2000, Hybridon may, at its option, redeem
the convertible preferred stock for cash equal to the Dividend Base Amount.

Class Voting Rights. Hybridon shall not, without the affirmative vote
or consent of the holders of at least 50% of all outstanding shares of
convertible preferred stock, voting separately as a class, (1) amend, alter or
repeal any provision of the restated certificate of incorporation or bylaws so
as adversely to affect the rights of the convertible preferred stock (except
that the issuance of securities ranking prior to, or pari passu with, the
convertible preferred stock (A) upon a Liquidation Event or (B) with respect to
the payment of dividends or distributions will not be considered to affect
adversely the relative rights of the convertible preferred stock), or (2)
authorize or issue, or increase the authorized amount of, the convertible
preferred stock, other than the convertible preferred stock issuable as
dividends on the convertible preferred stock.

Preemptive Rights. The convertible preferred stock is not entitled to 
any preemptive or subscription rights in respect of any securities of Hybridon.

Restrictions on Change of Control. So long as any of Hybridon's 9%
notes remain outstanding, no holder of any shares of convertible preferred stock
will, without the prior written consent of Hybridon, be granted voting rights,
be entitled to receive any voting securities of Hybridon, or be entitled to
exercise any conversion rights if that could, in Hybridon's reasonable judgment,
either alone or in conjunction with other issuances or holdings of capital
stock, warrants or convertible securities of Hybridon, result in a Change of
Control (as defined in the Indenture).

TRANSFER AGENT AND REGISTRAR

The transfer agent and registrar for the common stock is ChaseMellon
Shareholder Services LLC.

DELAWARE LAW AND CERTAIN PROVISIONS OF HYBRIDON'S RESTATED
CERTIFICATE OF INCORPORATION, BYLAWS AND INDEBTEDNESS

Hybridon is subject to the provisions of Section 203 of the Delaware
General Corporation Law, which prohibits a publicly-held Delaware corporation
from engaging in a "business combination" with an "interested stockholder" for a
period of three years after the date of the transaction in which the person
became an interested stockholder, unless the business combination is approved in
a prescribed manner. A "business combination" includes mergers, asset sales and
other transactions resulting in a financial benefit to the interested
stockholder. Subject to certain exceptions, an "interested stockholder" is a
person who, together with affiliates and associates, owns, or within three years
did own, 15% or more of the corporation's voting stock. The existence of this
provision could deter certain business combinations, including transactions that
might otherwise result in holders of voting stock being paid a premium over the
market price for their shares.

The restated certificate of incorporation provides for the division of
the board of directors into three classes as nearly equal in size as possible,
with the classes having staggered three-year terms. In addition, the restated
certificate of incorporation provides that directors may be removed only for
cause by the affirmative vote of the holders of at least two-thirds of the
shares of capital stock entitled to vote. Under the restated certificate of
incorporation, any vacancy on the board of directors, however occurring,
including a vacancy resulting from an enlargement of the board, may filled only
by vote of a majority of the directors then in office. The classification of the
board of directors and the limitations on the removal of directors and filling
of vacancies could have the effect of making it more difficult for anyone to
acquire, or of discouraging anyone from acquiring, control of Hybridon.

-56-

The restated certificate of incorporation also requires that any action
required or permitted to be taken by the stockholders of Hybridon at an annual
meeting or special meeting of stockholders may be taken only if it is properly
brought before that meeting and may not be taken by written action in lieu of a
meeting and will require reasonable advance notice by a stockholder of a
proposal or director nomination which that stockholder desires to present at any
annual or special meeting of stockholders. The restated certificate of
incorporation further provides that special meetings of the stockholders may be
called only by the Chief Executive Officer or, if none, the President of
Hybridon, or by the board of directors. Under Hybridon's bylaws, in order for
any matter to be considered "properly brought" before a meeting, a stockholder
must comply with certain requirements regarding advance notice to Hybridon. The
foregoing provisions could have the effect of delaying until the next
stockholders meeting any given stockholder action, even though it might be
favored by the holders of a majority of the outstanding voting securities of
Hybridon. These provisions may also discourage any person or entity from making
a tender offer for Shares of common stock, because such person or entity, even
if it acquired a majority of the outstanding voting securities of Hybridon,
would be able to take action as a stockholder (such as electing new directors or
approving a merger) only at a duly called stockholders meeting, and not by
written consent.

The Delaware General Corporation Law provides generally that the
affirmative vote of a majority of the shares entitled to vote on any matter is
required to amend a corporation's certificate of incorporation or bylaws, unless
a corporation's certificate of incorporation or by-law requires a greater
percentage. The restated certificate of incorporation and the bylaws require the
affirmative vote of the holders of at least 75% of the shares of capital stock
of Hybridon issued and outstanding and entitled to vote to amend or repeal any
of the provisions described in the prior two paragraphs. Moreover, the board of
directors has the authority, without further action by the stockholders, to fix
the rights and preferences of, and to issue shares of, any preferred stock other
than the convertible preferred stock.

In addition to these provisions of Delaware law, the restated
certificate of incorporation and the bylaws, the terms of Hybridon's outstanding
9% notes, which were issued in the aggregate original principal amount of $50.0
million and of which approximately $1.3 million in principal amount remains
outstanding, require Hybridon, upon a Change of Control of Hybridon (as defined
in the indenture for the 9% notes), to offer to repurchase the 9% notes at a
repurchase price equal to 150% of the principal amount thereof, plus accrued and
unpaid interest to the date of repurchase. This provision, together with the
provisions of the restated certificate of incorporation described above and
other provisions of the restated certificate of incorporation, may have the
effect of deterring takeovers or delaying or preventing changes in control or
management of Hybridon, including transactions in which stockholders might
otherwise receive a premium for their shares over then current market prices. In
addition, these provisions may limit the ability of stockholders to approve
transactions that they may deem to be in their best interests.

PLAN OF DISTRIBUTION

The securities offered in this prospectus may be sold from time to time
by the selling stockholders or their pledgees, donees, transferees or other
successors in interest. Sales of the securities may be effected on the NASD OTC
Bulletin Board or in negotiated transactions at prices then prevailing or
related to the then-current market price, or at negotiated prices.

The securities may be sold directly or through brokers or dealers by
means of one or more of the following methods: (i) block trades in which the
broker or dealer attempts to sell shares as agent but may position and resell a
portion of the block as principal to facilitate the transaction; (ii) purchases
by a broker or dealer as principal and resales by that broker or dealer for its
own account pursuant to this prospectus, including resale to another broker or
dealer; and (iii) ordinary brokerage transactions and transactions in which the
broker solicits purchasers. In effecting sales, brokers and dealers engaged by
selling stockholders may arrange for other brokers or dealers to participate.
Brokers or dealers may receive commissions or discounts from selling
stockholders (or, if any such broker or dealer acts as agent for the purchaser
of any securities, from that purchaser) in amounts to be negotiated. A
broker-dealer may agree with the selling stockholders to sell a specified number
of securities at a stipulated price per share, and, to the extent that
broker-dealer is unable to do so acting as agent for the selling stockholders,
to purchase as principal any unsold securities at the price required to fulfill
the broker-dealer commitment to the selling stockholders. Broker-dealers who
acquire securities as principal may thereafter resell those securities.

The selling stockholders and any broker-dealers participating in
distribution of the securities may be deemed "underwriters" within the meaning
of Section 2(11) of the Securities Act, and any profit on the sale of securities
by the selling stockholders and any commissions or discounts given to
broker-dealers may be deemed 

-57-

underwriting commissions or discounts under the Securities Act. In addition, any
of the securities that qualify for sale pursuant to Rule 144 may be sold under
Rule 144 rather than pursuant to this prospectus.

Hybridon has agreed to indemnify certain of the selling stockholders,
each underwriter of certain of the securities, and each person controlling
certain of the selling stockholders within the meaning of Section 15 of the
Securities Act, against certain liabilities in connection with the offer and
sale of the securities, including liabilities under the Securities Act, and to
contribute to payments those persons may be required to make in respect of such
liabilities. Certain of the selling stockholders have agreed to indemnify, in
certain circumstances, Hybridon against certain liabilities in connection with
the offer and sale of the securities, including liabilities under the Securities
Act, and to contribute to payments Hybridon may be required to make in respect
thereof.

LEGAL MATTERS

The validity of the securities offered by this prospectus will be
passed upon for Hybridon by Kramer Levin Naftalis & Frankel LLP, New York, New
York.

EXPERTS

The consolidated financial statements of Hybridon as of December 31,
1996, 1997, and 1998 and for each of the years in the three-year period ended
December 31, 1998 included in this prospectus and elsewhere in the registration
statement have been audited by Arthur Andersen LLP, independent public
accountants, as indicated in their report with respect thereto, which report
includes a paragraph stating that there is substantial doubt about Hybridon's
ability to continue as a going concern, and are included herein in reliance upon
the authority of said firm as experts in accounting and auditing.

ADDITIONAL INFORMATION

We have filed a registration statement on Form S-1 with the Securities
and Exchange Commission relating to the common stock offered by this prospectus.
This prospectus does not contain all of the information set forth in the
registration statement and the exhibits and schedules to the registration
statement. Statements contained in this prospectus concerning the contents of
any contract or other document referred to are not necessarily complete and in
each instance we refer you to the copy of the contract or other document filed
as an exhibit to the registration statement, each such statement being qualified
in all respects by such reference.

For further information with respect to us and the common stock offered
in this prospectus, please refer to the registration statement. A copy of the
registration statement can be inspected by anyone without charge at the public
reference room of the SEC, Room 1024, 450 Fifth Street, N.W., Washington, D.C.
20549, and at the SEC's Regional Offices located at 7 World Trade Center, Suite
1300, New York, New York 10048, and 500 West Madison Street, Chicago, Illinois
60601. Please call the SEC at 1-800-SEC-0330 for further information on the
operation of the public reference room. Copies of these materials can be
obtained by mail from the Public Reference Section of the SEC at 450 Fifth
Street, N.W., Washington, D.C. 20549, at prescribed rates. The SEC maintains a
Web site (http://www.sec.gov) that contains information regarding registrants
that file electronically with the SEC.

-58-

HYBRIDON, INC. AND SUBSIDIARIES
INDEX TO CONSOLIDATED FINANCIAL STATEMENTS

PAGE

REPORT OF INDEPENDENT PUBLIC ACCOUNTANTS                                     F-2

CONSOLIDATED BALANCE SHEETS AS OF DECEMBER 31, 1997,
DECEMBER 31, 1998 AND SEPTEMBER 30, 1999 (UNAUDITED)                         F-3

CONSOLIDATED STATEMENTS OF OPERATIONS FOR THE YEARS ENDED DECEMBER 31, 1996,
DECEMBER 31, 1997 AND DECEMBER 31, 1998 AND FOR THE NINE MONTHS ENDED
SEPTEMBER 30, 1998 AND SEPTEMBER 30, 1999 (UNAUDITED)                        F-4

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT) FOR THE YEARS 
ENDED DECEMBER 31, 1996, DECEMBER 31, 1997 AND DECEMBER 31, 1998 AND 
FOR THE NINE MONTHS ENDED SEPTEMBER 30, 1999 (UNAUDITED)                     F-5

CONSOLIDATED STATEMENTS OF CASH FLOWS FOR THE YEARS ENDED DECEMBER 31, 1996,
DECEMBER 31, 1997 AND DECEMBER 31, 1998 AND FOR THE NINE MONTHS ENDED
SEPTEMBER 30, 1998 AND SEPTEMBER 30, 1999 (UNAUDITED)                        F-7

NOTES TO CONSOLIDATED FINANCIAL STATEMENTS                                   F-8

F-1

REPORT OF INDEPENDENT PUBLIC ACCOUNTANTS

To Hybridon, Inc.:

We have audited the accompanying consolidated balance sheets of Hybridon, Inc.
(a Delaware corporation) and subsidiaries as of December 31, 1997 and 1998, and
the related consolidated statements of operations, stockholders' equity
(deficit) and cash flows for each of the three years in the period ended
December 31, 1998. These financial statements are the responsibility of the
Company's management. Our responsibility is to express an opinion on these
financial statements based on our audits.

We conducted our audits in accordance with generally accepted auditing
standards. Those standards require that we plan and perform the audit to obtain
reasonable assurance about whether the financial statements are free of material
misstatement. An audit includes examining, on a test basis, evidence supporting
the amounts and disclosures in the financial statements. An audit also includes
assessing the accounting principles used and significant estimates made by
management, as well as evaluating the overall financial statement presentation.
We believe that our audits provide a reasonable basis for our opinion.

In our opinion, the financial statements referred to above present fairly, in
all material respects, the financial position of Hybridon, Inc. and subsidiaries
as of December 31, 1997 and 1998 and the consolidated results of their
operations and their cash flows for each of the three years in the period ended
December 31, 1998, in conformity with generally accepted accounting principles.

The accompanying consolidated financial statements have been prepared assuming
that the Company will continue as a going concern. Since inception, the Company
has incurred significant losses which it has funded through the issuance of debt
and equity securities and through research and development collaborations and
licensing agreements. The Company expects such resources to fund operations
through May 1999. There is substantial doubt about the Company's ability to
continue as a going concern. See Note 1 for management's plans. The financial
statements do not include any adjustments that might result from the outcome of
this uncertainty.

/s/ ARTHUR ANDERSEN LLP

Boston, Massachusetts
February 19, 1999 (except with respect to the matter 
discussed in Note 7(b) as to which the date is April 15, 1999)

F-2

HYBRIDON, INC. AND SUBSIDIARIES
CONSOLIDATED BALANCE SHEETS

ASSETS
September 30,
December 31,                    1999
1997               1998          (unaudited)
------------------- ----------------- -----------------

CURRENT ASSETS:
Cash and cash equivalents                                             $        2,202,202  $      5,607,882  $     500,179
Accounts receivable                                                              529,702         1,175,441        838,852
Prepaid expenses and other current assets                                      1,005,825           110,827        102,185
------------------  ----------------  -------------

Total current assets                                                       3,737,729         6,894,150      1,441,216
------------------  ----------------  -------------

PROPERTY AND EQUIPMENT, AT COST:
Leasehold improvements                                                        16,027,734        11,127,035     11,127,035
Laboratory and other equipment                                                14,288,083        11,432,435      9,988,579
------------------  ----------------  -------------
30,315,817        22,559,470     21,115,614

Less--Accumulated depreciation and amortization                                11,085,013        13,788,979     14,162,190
------------------  ----------------  -------------

19,230,804         8,770,491      6,953,424
------------------  ----------------  -------------
OTHER ASSETS:
Deferred financing costs and other assets                                      3,354,767           612,374        531,423
Note receivable from officer                                                     247,250           258,650        267,200
Restricted cash                                                                3,050,982                --             --
Investment in real estate partnership                                          5,450,000                --             --
------------------  ----------------  -------------
12,102,999           871,024        798,623
------------------  ----------------  -------------

$       35,071,532  $     16,535,665  $   9,193,263
==================  ================  ===============

LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
CURRENT LIABILITIES:
Current portion of long-term debt                                 $       7,868,474   $      6,070,951  $   6,078,179
Related party promissory notes payable                                           --                 --      1,000,000
Accounts payable                                                          8,051,817          2,368,163      2,512,738
Accrued expenses                                                         11,917,298          4,068,679      2,389,804
-----------------   ----------------  -------------

Total current liabilities                                       27,837,589         12,507,793     11,980,721
-----------------   ----------------  -------------

LONG-TERM DEBT, NET OF CURRENT PORTION                                             3,282,123            473,094        413,523
-----------------   ----------------  -------------

9% CONVERTIBLE SUBORDINATED NOTES PAYABLE                                         50,000,000          1,306,000      1,306,000
-----------------   ----------------  -------------

COMMITMENTS AND CONTINGENCIES (Notes 11 and 16)

STOCKHOLDERS' EQUITY (DEFICIT):
Preferred stock, $.01 par value-
Authorized--5,000,000 shares
Series A convertible preferred stock-
Designated--1,500,000 shares
Issued and outstanding--641,259 shares at                                   --              6,413          6,410
December 31, 1998 and 641,023 shares at September 30, 1999
(Liquidation preference of $65,178,199 at September 30, 1999)
Common stock, $.001 par value-
Authorized--100,000,000 shares
Issued and outstanding--5,059,650 shares at December 31,
1997 and 15,304,825 at December 31, 1998                                  5,060             15,305         16,261
and 16,260,722 shares at September 30, 1999 (unaudited),
respectively
Additional paid-in capital                                                  173,695,698        241,632,024    246,227,811
Accumulated deficit                                                        (218,655,101)      (238,447,837)  (249,974,144)
Deferred compensation                                                        (1,093,837)          (957,127)      (783,319)
------------------  ------------------ -----------
Total stockholders' (deficit) equity                                    (46,048,180)         2,248,778     (4,506,981)
------------------  ----------------  -------------
$    35,071,532   $     16,535,665  $   9,193,263
===============   ================  =============

The accompanying notes are an integral part of these
consolidated financial statements.

F-3

HYBRIDON, INC. AND SUBSIDIARIES
CONSOLIDATED STATEMENTS OF OPERATIONS

Nine Months Ended
Years Ended December 31,                      September 30,
1996            1997              1998            1998            1999
--------------------------------------------------------------------------------
(unaudited)
REVENUES:

Product and service                $  1,080,175    $  1,876,862    $  3,253,879    $  2,353,435    $  4,643,842
Research and development              1,419,389         945,000       1,099,915         949,916         450,000
Royalty and other income                 62,321          48,000              --              --         106,950
Interest                              1,446,762       1,079,122         148,067         106,457          81,724
------------    ------------    ------------    ------------    ------------
4,008,647       3,948,984       4,501,861       3,409,807       5,282,516
------------    ------------    ------------    ------------    ------------
OPERATING EXPENSES:
Research and development             39,390,525      46,827,915      20,977,370      17,180,927      10,106,459
General and administrative           11,346,670      11,026,748       6,572,502       5,817,864       2,946,564
Interest                                124,052       4,535,647       2,932,362       2,880,307         561,949
Restructuring                                 --      11,020,000              --              --              --
------------    ------------   ------------    ------------    ------------
Total operating expenses            50,861,247      73,410,310      30,482,234      25,879,098      13,614,972
------------    ------------   ------------    ------------    ------------
Loss  before  extraordinary item   (46,852,600)    (69,461,326)    (25,980,373)    (22,469,291)     (8,332,456)

EXTRAORDINARY ITEM:
Gain on exchange of 9%                       --              --       8,876,685       8,876,685              --
convertible subordinated notes 
payable                             ------------    ------------   ------------    ------------    ------------
Net loss                            (46,852,600)    (69,461,326)    (17,103,688)    (13,592,606)     (8,332,456)
============    ============   =============   =============    ============
ACCRETION OF PREFERRED STOCK              
DIVIDENDS                                         --              --       2,689,048       1,647,000       3,193,851
------------    ------------   -------------   ------------     -----------
Net loss applicable to
common stockholders          $(46,852,600)   $(69,461,326    $(19,792,736    $(15,239.606)   $(11,526,307)
=============   ============    ============    =============   =============
BASIC AND DILUTED NET LOSS PER
COMMON SHARE:

Loss per share before          $     (10.24)   $     (13.76)   $      (2.19)   $      (2.11)   $      (0.54)
extraordinary item    

Extraordinary item                       --              --            0.75            0.83              --
-------------   -------------   -------------   -------------   -------------
Net loss per share                   (10.24)         (13.76)          (1.44)          (1.28)          (0.54)

Accretion of preferred stock             --              --            (.23)          (0.15)          (0.20)
dividends
-------------   -------------   -------------   -------------   -------------
Net loss per share applicable
to common stockholders      $      (10.24)   $     (13.76)  $       (1.67)   $      (1.43)   $      (0.74)
-------------   -------------   -------------   -------------   -------------
SHARES USED IN COMPUTING BASIC
AND DILUTED NET LOSS PER COMMON SHARE      4,575,555       5,049,840      11,859,350      10,648,116      15,653,562
==============   =============  ==============   =============   ============

The accompanying notes are an integral part of these
consolidated financial statements.

F-4

HYBRIDON, INC. AND SUBSIDIARIES
CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT)

Convertible           Series A Convertible     Common Stock
Preferred Stock          Preferred Stock

Number of      $.01 Par    Number of  $.01 Par   Number of    $.001 Par   
Shares         Value       Shares    Value     Shares        Value      

BALANCE, DECEMBER 31, 1995                              3,196,435    $  31,965      --       $ --     368,733         $    369  
Issuance of common stock related to initial                                                                                 
public offering, net of issuance                                                                                        
costs of $5,268,756                                  --           --      --         --   1,150,000            1,150  
Conversion of convertible preferred stock to                                                                                
common stock                                   (3,196,435)     (31,965)     --         --   3,371,330            3,371  
Issuance of common stock related  to  the                                                                                   
exercise of stock options                              --           --      --         --      57,740               58  
Issuance of common stock  related  to  the                                                                                  
exercise of  warrants                                  --           --      --         --      81,512               81  
Deferred  compensation related to grants of                                                                                 
stock options to nonemployees                          --           --      --         --          --               --  
Amortization of deferred compensation                      --           --      --         --          --               --  
Net loss                                                   --           --      --         --          --               --  
-----------    ---------  ------   --------   ---------        ---------

BALANCE, DECEMBER 31, 1996                                     --           --      --         --   5,029,315            5,029  
Issuance of common stock related to the                                                                                     
exercise of stock options                              --           --      --         --          --           25,005  
Issuance of common stock related  to  the                                                                                   
exercise of warrants                                   --           --      --         --         330               --  
Issuance of common stock for services                                                                                       
rendered                                               --           --      --         --       5,000                5  
Deferred  compensation  related  to grants of                                                                               
stock options to nonemployees                              --           --      --         --          --               --  
Amortization of deferred compensation                      --           --      --         --          --               --  
Net loss                                                   --           --      --         --          --               --  
-----------    ---------  ------   --------   ---------        ---------
BALANCE, DECEMBER 31, 1997                                     --           --      --         --   5,059,650            5,060  

Total     
Additional     Accumulated         Deferred        Stockholders' 
Paid-in         Deficit         Compensation        Equity     
Capital                                            (Deficit)   

BALANCE, DECEMBER 31, 1995                          $ 114,755,394   $(102,341,175)   $          --    $  12,446,553
Issuance of common stock related to initial
public offering, net of issuance
costs of $5,268,756                          52,230,094              --               --       52,231,244
Conversion of convertible preferred stock to
common stock                                       28,594              --               --               --
Issuance of common stock related  to  the
exercise of stock options                       1,089,618              --               --        1,089,676
Issuance of common stock  related  to  the
exercise of  warrants                           3,176,660              --               --        3,176,741
Deferred  compensation related to grants of
stock options to nonemployees                   1,967,116              --       (1,967,116)              --
Amortization of deferred compensation                      --              --          763,190          763,190
Net loss                                                   --     (46,852,600)              --      (46,852,600)
-----------    ------------       ----------       ----------

BALANCE, DECEMBER 31, 1996                            173,247,476    (149,193,775)      (1,203,926)      22,854,804
Issuance of common stock related to the
exercise of stock options                              26          86,300               --           86,326
Issuance of common stock related  to  the
exercise of warrants                                9,075              --               --            9,075
Issuance of common stock for services
rendered                                          146,869              --               --          146,874
Deferred  compensation  related  to grants of
stock options to nonemployees                         205,978              --         (205,978)              --
Amortization of deferred compensation                      --              --          316,067          316,067
Net loss                                                   --     (69,461,326)              --      (69,461,326)
-----------    ------------       ----------       ----------

BALANCE, DECEMBER 31, 1997                            173,695,698    (218,655,101)      (1,093,837)     (46,048,180)

F-5

HYBRIDON, INC. AND SUBSIDIARIES
CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT)
(Continued)

Convertible       Series A Convertible           Common Stock
Preferred Stock       Preferred Stock

Number of     $.01 Par  Number of    $.01 Par     Number of       $.001 Par    
Shares       Value     Shares       Value        Shares           Value      

BALANCE, DECEMBER 31, 1997                             --          --           --            --        5,059,650        5,060 
Issuance of Series A convertible  preferred stock                                                                            
and attached warrants in exchange for                                                                                     
conversion of 9% convertible subordinated                                                                                 
notes  payable and accrued interest,  net                                                                                 
of issuance costs of $1,195,398                   --          --      510,504         5,105               --           -- 
Issuance of common stock and attached                                                                                        
warrants in exchange for conversion of accounts                                                                           
payable and other obligations                     --          --           --            --        3,217,154        3,217 
Issuance of Series A convertible preferred stock     --          --      114,285         1,143               --           -- 
Issuance of common stock to Placement Agent          --          --           --            --          597,699          598 
Issuance of common stock and attached                                                                                        
warrants in exchange for conversion of                                                                                    
convertible notes  payable, net of issuance                                                                               
costs of $566,167                                 --          --           --            --        3,157,322        3,157 
Issuance of common stock and attached                                                                                        
warrants, net of issuance costs of $1,069,970     --          --           --            --        3,223,000        3,223 
Issuance of common stock for services                --          --           --            --           50,000           50 
rendered                                                                                                                  
Deferred compensation related to grants of                                                                                   
stock options to nonemployees, net  of                                                                                    
terminations                                      --          --           --            --               --           -- 
Issuance of warrants in connection with                                                                                      
notes payable                                     --          --           --            --               --           -- 
Accretion and issuance of Series A                                                                                           
convertible preferred stock dividends             --          --       16,470           165               --           -- 
Amortization of deferred compensation             --          --           --            --               --           -- 
Net loss                                                      --           --            --               --           -- 
=====        ====      =======    ==========       ==========      ======= 
BALANCE, DECEMBER 31, 1998                             --          --      641,259         6,413       15,304,825       15,305 
Issuance of common stock to placement agents         --          --           --            --          460,000          460 
Amortization of deferred compensation                --          --           --            --               --           -- 
Compensation expense related to                                                                                              
grants of stock options to nonemployees           --          --           --            --               --           -- 
Accretion and issuance of Series A convertible                                                                               
preferred stock dividend                          --          --       20,840           208               --           -- 
Conversion of Series A convertible  preferred                                                                                
stock  into common stock                          --          --      (21,076)         (211)         495,897          496 
Net loss                                             --          --           --            --               --           -- 
=====        ====      =======    ==========       ==========      ======= 
BALANCE, SEPTEMBER 30, 1999 (UNAUDITED)                --          --      641,023    $    6,410       16,260,722      $16,261 
=====        ====      =======    ==========       ==========      ======= 

Total
Additional      Accumulated          Deferred       Stockholders'  
Paid-in          Deficit          Compensation        Equity      
Capital                                             (Deficit)     

BALANCE, DECEMBER 31, 1997                                173,695,698     (218,655,101)      (1,093,837)     (46,048,180)
Issuance of Series A convertible  preferred stock
and attached warrants in exchange for
conversion of 9% convertible subordinated
notes  payable and accrued interest,  net
of issuance costs of $1,195,398                       38,729,489               --               --       38,734,594
Issuance of common stock and attached
warrants in exchange for conversion of accounts
payable and other obligations                          5,931,341               --               --        5,934,558
Issuance of Series A convertible preferred stock          7,998,817               --               --        7,999,960
Issuance of common stock to Placement Agent               1,194,800               --               --        1,195,398
Issuance of common stock and attached
warrants in exchange for conversion of
convertible notes  payable, net of issuance
costs of $566,167                                      4,230,676               --               --        4,233,833
Issuance of common stock and attached
warrants, net of issuance costs of $1,069,970          6,873,453               --               --        6,876,676
Issuance of common stock for services                        93,700               --               --           93,750
rendered
Deferred compensation related to grants of
stock options to nonemployees, net  of
terminations                                             109,734               --         (109,734)              --
Issuance of warrants in connection with
notes payable                                             85,433               --               --           85,433
Accretion and issuance of Series A
convertible preferred stock dividends                  2,688,883       (2,689,048)              --               --
Amortization of deferred compensation                         --               --          246,444          246,444
Net loss                                                      --      (17,103,688)              --      (17,103,688)
=============    =============    =============    ============= 
BALANCE, DECEMBER 31, 1998                                241,632,024     (238,447,837)        (957,127)       2,248,778
Issuance of common stock to placement agents                999,540               --               --        1,000,000
Amortization of deferred compensation                            --               --          173,808          173,808
Compensation expense related to
grants of stock options to nonemployees                  402,889               --               --          402,889
Accretion and issuance of Series A convertible
preferred stock dividend                               3,193,643       (3,193,851)              --               --
Conversion of Series A convertible  preferred
stock  into common stock                                    (285)              --               --               --
Net loss                                                         --       (8,332,456)              --       (8,332,456)
=============    =============    =============    ============= 
BALANCE, SEPTEMBER 30, 1999 (UNAUDITED)                 $ 246,227,811    $ 249,974,144    $    (783,319)   $  (4,506,981)
=============    =============    =============    ============= 

The accompanying notes are an integral part of these consolidated
financial statements.

F-6

HYBRIDON, INC. AND SUBSIDIARIES
CONSOLIDATED STATEMENTS OF CASH FLOWS

Years Ended December 31,      Nine Months Ended September 30,
1996          1997          1998         1998             1999
CASH FLOWS FROM OPERATING ACTIVITIES:                                                                           (Unaudited)

Net loss                                                $(46,852,600) $(69,461,326)  $(17,103,688)   $(13,592,606) $ (8,332,456)
Adjustments to reconcile net loss to net
cash used in operating activities-
Extraordinary gain on exchange of 9%
convertible subordinated notes payable                       --            --     (8,876,685)     (8,876,685)           --
Depreciation and amortization                           2,393,751     4,488,719      4,057,286       2,419,269     1,825,370
Loss on disposal of fixed assets                               --            --             --         424,675            --
Issuance of common stock for services rendered                 --       146,874         93,750              --            --
Amortization of deferred compensation                     763,190       316,067        246,444         163,044       576,697
Amortization of deferred financing costs                       --       479,737        160,813         240,611        80,951
Noncash portion of restructuring charge                        --     1,255,000             --              --            --
Changes in assets and liabilities-
Accounts receivable                                   (573,896)       44,194       (645,739)       (295,966)      336,589
Prepaid expenses and other current assets             (593,797)      539,499        894,998         557,703         8,642
Note receivable from officer                            (9,845)       70,728        (11,400)         (8,550)       (8,550)
Accounts payable                                     2,010,981     3,987,398     (3,059,002)       (377,733)      144,575
Accrued expenses                                       736,141     7,071,532      1,565,806         706,406      (678,875)
Deferred revenue                                            --       (86,250)            --              --            --
Amounts payable to related parties                     (12,500)           --             --              --            --

Net cash used in operating activities           (42,138,575)  (51,147,828)   (22,677,417)    (18,639,832)   (6,047,057)

CASH FLOWS FROM INVESTING ACTIVITIES:
(Increase) decrease in short-term investments             (3,785,146)    3,785,146             --              --            --
Purchases of property and equipment                       (8,902,989)   (7,509,755)      (471,949)       (340,507)           --
Proceeds from sale of property and equipment                      --            --        714,400         460,000        (8,303)
(Investment in) sale of real estate partnership           (3,751,552)           --      5,450,000              --            --

Net cash (used in) provided by investing
activities                                    (16,439,687)   (3,724,609)     5,692,451         119,493        (8,303)

CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of Series A convertible
preferred stock                                                 --            --      7,999,960       7,999,960            --
Proceeds from issuance of common stock related
to stock options and restricted stock grants             1,089,676        86,326             --              --            --

Net proceeds from issuance of common stock                52,231,244            --      6,876,676       6,876,676            --
Proceeds from notes payable                                7,500,000            --      6,000,000              --            --
Proceeds from issuance of convertible promissory
notes payable                                                   --    50,000,000      4,233,833       4,233,833            --

Proceeds from related party promissory notes payable              --            --             --              --     1,000,000
Proceeds from issuance of common stock related to          3,176,741         9,075             --              --            --
stock warrants
Proceeds from sale/leaseback of fixed assets               1,722,333     1,205,502             --              --            --
Payments on long-term debt                                  (446,163)   (1,564,268)    (7,296,646)     (4,236,693)      (52,343)
Decrease (increase) in deferred financing costs              251,921    (2,820,790)      (400,000)             --            --
Decrease (increase) in restricted cash and other assets      401,990    (2,474,948)     2,976,823       2,327,186            --

Net cash provided by financing activities        65,927,742    44,440,897     20,390,646      17,200,962       947,657

NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS           7,349,480   (10,431,540)     3,405,680      (1,319,377)   (5,107,703)
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR                   5,284,262    12,633,742      2,202,202       2,202,202     5,607,882
CASH AND CASH EQUIVALENTS, END OF YEAR                      $ 12,633,742  $  2,202,202   $  5,607,882    $    882,825  $    500,179

The accompanying notes are an integral part of these consolidated
financial statements.

F-7

HYBRIDON, INC. AND SUBSIDIARIES
NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
(Including Data Applicable to Unaudited Periods)

(1)    ORGANIZATION

Hybridon, Inc. (the Company) was incorporated in the state of Delaware on
May 25, 1989. The Company is engaged in the discovery and development of
novel genetic medicines based primarily on antisense technology.

Since inception, the Company has devoted substantially all of its efforts
toward product research and development, its custom contract
manufacturing business (Hybridon Specialty Products or HSP) and raising
capital. Management anticipates that substantially all future revenues
will be derived from the sale of proprietary biopharmaceutical products
under development or to be developed in the future, and custom contract
manufacturing of synthetic DNA products and reagent products (by HSP), as
well as from research and development revenues and fees and royalties
derived from licensing of the Company's technology. Accordingly, although
the Company has begun to generate revenues from its custom contract
manufacturing business, the Company is dependent on the proceeds from
possible future sales of debt and equity securities and research and
development collaborations in order to fund future operations. There is
substantial doubt concerning its ability to continue as a going concern.
As of December 31, 1998, the Company had cash and cash equivalents of
approximately $5.6 million. The Company expects such resources to fund
operations through May 1999. The consolidated financial statements do not
include any adjustments that might result from the outcome of this
uncertainty.

The Company is currently seeking debt or equity financing in an amount
sufficient to support its operations through the end of 1999, and in
connection therewith, is in negotiations with several parties to obtain
such financing. If the Company is unable to obtain this sufficient amount
of additional funding in May 1999, it will be forced to terminate its
operations or seek relief under applicable bankruptcy law by the end of
May 1999.

See Note 22 for additional information through February 1, 2000.

On December 3, 1997, the Company was delisted from the Nasdaq Stock
Market, Inc. (NASDAQ) because the Company was not in compliance with the
continued listing requirements of the NASDAQ National Market. The Company
is currently trading on the NASD OTC as a result of the delisting.

(2)    SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

(a)    Management Estimates and Uncertainties

The preparation of financial statements in conformity with
generally accepted accounting principles requires management to
make estimates and assumptions that affect the reported amounts of
assets and liabilities and disclosure of contingent assets and
liabilities at the date of the financial statements and the
reported amounts of revenues and expenses during the reporting
period. Actual results could differ from those estimates.

The Company is subject to a number of risks and uncertainties
similar to those of other companies of the same size within the
biotechnology industry, such as uncertainty with clinical trials,
uncertainty of additional funding and history of operating losses.

F-8

(b)    Principles of Consolidation

The accompanying consolidated financial statements include the
results of the Company and its subsidiaries, Hybridon S.A.
(Europe), a French corporation, and Hybridon Canada, Inc. (an
inactive majority-owned subsidiary). The consolidated financial
statements also reflect the Company's 30% interest in MethylGene,
Inc. (MethylGene), a Canadian corporation which is accounted for
under the equity method (see Note 14). All material intercompany
balances and transactions have been eliminated in consolidation.

(c)    Cash Equivalents

The Company considers all highly liquid investments with
maturities of three months or less when purchased to be cash
equivalents. Cash and cash equivalents and restricted cash at
December 31, 1997 and 1998 consisted of the following (at
amortized cost, which approximates fair market value):

1997          1998
----------    ----------
Cash and cash equivalents-
Cash and money market funds        $1,702,272    $3,865,365
Corporate bond                        499,930     1,742,517
----------   -----------
Total cash and cash equivalents    $2,202,202    $5,607,882
==========    ==========

Restricted cash-
Note payable to bank (Note 7(a))   $1,758,542    $        -
Foreign bank account (Note 6)       1,034,618             -
Capital lease obligations 
(Note 7(d))                           257,822             -
----------    ----------
$3,050,982    $        -
==========    ==========

(d)    Depreciation and Amortization

Depreciation and amortization are computed using the straight-line
method based on the estimated useful lives of the related assets
as follows:

Estimated
Asset Classification          Useful Life

Leasehold improvements                 Life of lease
Laboratory equipment and other         3-5 years

(e)    Accrued Expenses

At December 31, 1997 and 1998, accrued expenses consist of the
following:

F-9

1997             1998
-----------     -------------

Restructuring (Note 3)                  $  8,316,148     $    469,485
Interest                                   1,125,000           29,385
Payroll and related costs                    742,452        1,151,742
Outside research and clinical costs        1,231,818          797,593
Professional fees                            150,000          149,957
Contingent stock (Notes 7(b) and 15(c))            -        1,000,000
Other                                        351,880          470,517
------------     ------------
$11,917,298       $4,068,679

(f)    Reclassifications

Certain amounts in the prior periods consolidated financial
statements have been reclassified to conform with the current
period's presentation.

(g)    Revenue Recognition

The Company has recorded revenue under the consulting and research
agreements discussed in Notes 8, 9 and 14. Revenue is recognized
as earned on a straight-line basis over the term of the agreement,
which approximates when work is performed and costs are incurred.
Revenues from product and service sales are recognized when the
products are shipped or the services are performed. Product
revenue during 1997 and 1998 represents revenues from the sale of
oligonucleotides manufactured on a custom contract basis by HSP.
Revenue from related parties totaled $50,000, $102,000,
$1,686,000, $1,600,000 and $976,000 for 1996, 1997 and 1998 and
the nine months ended September 30, 1998 and 1999, respectively.

(h)    Research and Development Expenses

The Company charges research and development expenses to
operations as incurred.

(i)    Patent Costs

The Company charges patent expenses to operations as incurred.

(j)    Comprehensive Loss

The Company applies Statement of Financial Accounting Standards
(SFAS) No. 130, Reporting Comprehensive Income. Comprehensive loss
is defined as the change in equity of a business enterprise during
a period from transactions and other events and circumstances from
nonowner sources. The Company's comprehensive loss is the same as
the reported net loss for all periods presented.

(k)    Net Loss per Common Share

The Company applies SFAS No. 128, Earnings per Share. Under SFAS
No. 128, basic net loss per common share is computed using the
weighted average number of shares of common stock outstanding
during the period. Diluted net loss per common share is the same
as basic net loss per common share as the effects of the Company's
potential common stock

F-10

equivalents are antidilutive. Antidilutive securities which
consist of stock options, warrants and convertible preferred stock
(on an as-converted basis) that are not included in diluted net
loss per common share were 2,595,496, 2,404,561 and 27,774,883 for
1996, 1997 and 1998, respectively.

(l)    Segment Reporting

The Company applies SFAS No. 131, Disclosures about Segments of an
Enterprise and Related Information. SFAS No. 131 establishes
standards for reporting information regarding operating segments
in annual financial statements and requires selected information
for those segments to be presented in interim financial reports
issued to stockholders. SFAS No. 131 also establishes standards
for related disclosures about products and services and geographic
areas. To date, the Company has viewed its operations and manages
its business as principally one operating segment. As a result,
the financial information disclosed herein, represents all of the
material financial information related to the Company's principal
operating segment. All of the Company's revenues are generated in
the U.S. and substantially all assets are located in the U.S.

(3)    RESTRUCTURING

Beginning in July 1997, the Company implemented a restructuring plan to
reduce expenditures on a phased basis in an effort to conserve its cash
resources. As part of this restructuring plan, in addition to terminating
the clinical development of GEM(R) 91, the Company's first generation
antisense drug for the treatment of AIDS and HIV infection, the Company
reduced or suspended programs unrelated to its core advanced chemistry
antisense drug research and development programs. In connection with the
reduction in programs, the Company has accrued termination fees related
to research contracts and has written off assets related to programs that
have been suspended or canceled. As part of the restructuring, all
outside testing, public relations, travel and entertainment and
consulting arrangements were reviewed and where appropriate the terms
were renegotiated, contracts cancelled or the terms significantly
reduced. As a result of the implementation of these changes, the Company
terminated the employment of 84 employees at its Cambridge and Milford,
Massachusetts, facilities in 1997 and closed its operations in Paris,
France, terminating 11 employees at that location.

In connection with the restructuring, the Company entered into different
subleasing arrangements. During 1997, the Company subleased a portion of
each of its facilities in Cambridge, Massachusetts (including a
substantial portion of its former headquarters located at 620 Memorial
Drive (the Cambridge Headquarters)). The Company incurred expenses
relating to these subleases for broker fees and renovation expenses
incurred in preparing the Cambridge Headquarters space for the new
tenant. In addition, the Company accrued the estimated lease loss of
subleasing the Cambridge Headquarters which were vacated during 1998. The
Company also subleased its office in Paris, France, and accrued the
estimated lease loss.

The following are the significant components of the $11,020,000 charge
for restructuring (in thousands):

To be Paid
as of
Restructuring      Non-Cash         Cash       December 31,
Charge          Portion        Disbursed        1998
---------------- --------------- -------------- ------------

Estimated loss on facility leases        $    6,372       $    5,976      $      356     $       40
Employee severance, benefits and              2,738                -           2,548            190
related costs
Write-down of assets to net realizable          946              946               -              -
value
Termination costs of certain                    964              672              53            239
----------       ----------      ----------     ----------
research programs
$   11,020       $    7,594      $    2,957     $      469
==========       ==========      ==========     ==========

F-11

The Company disbursed cash totaling approximately $1,453,000 and
$1,504,000 in 1997 and 1998, respectively, with respect to the
restructuring. The remaining accrued amount of approximately $469,000
will be paid during 1999.

(4)    INVESTMENT IN REAL ESTATE PARTNERSHIP

Under the terms of the lease for the Cambridge Headquarters (the
Cambridge Lease), the Company accounted for $5,450,000 of its payments
for a portion of the costs of construction of the leased premises as
contributions to the capital of the Cambridge landlord in exchange for a
limited partnership interest in the Cambridge landlord (the Partnership
Interest). Under the terms of the Partnership Interest, the Company
exercised its right to sell back the Partnership Interest and received
payment of the $5,450,000 in 1998.

(5)    NOTE RECEIVABLE FROM OFFICER

At December 31, 1997 and 1998 the Company has a note receivable from
officer, including accrued interest, of $247,250 and $258,650,
respectively. The note has an interest rate of 6.0% per annum and matures
in April 2001.

(6)    RESTRICTED CASH - BVH

In November 1997, the Company was notified by Bank Fur Vermogensanlagen
Und Handel AG (BVH) that the Federal Banking Supervisory Office in
Germany had imposed a moratorium on BVH and had closed BVH for business.
Accordingly, the Company classified its deposit with BVH as restricted
cash. The Company sold the deposit to the Cambridge Landlord, an
affiliate of certain directors of the Company, and recovered the full
amount in 1998.

(7)    LONG-TERM DEBT AND CAPITAL LEASE OBLIGATIONS

Future minimum principal payments due under various notes payable,
excluding the 9% convertible subordinated notes (the 9% Notes) due April
1, 2004, are as follows at December 31, 1998:

December 31,                       Amount
------------                       ------

1999                      $  6,070,951
2000                            80,746
2001                            91,892
2002                           104,576
2003                           119,010
Thereafter                        76,870
------------
Total long-term debt obligations         6,544,045

Less--Current portion                    6,070,951

$    473,094

(a)    Note Payable to a Bank

F-12

In December 1996, the Company entered into a five-year $7,500,000
note payable to a bank. In November 1998, the outstanding balance
of approximately $2,895,000 was purchased from the bank by Forum
Capital Markets, LLC (Forum) and certain investors associated with
Pecks Management Partners Ltd. (Pecks) (collectively, the
Lenders), which are affiliates of two members of the Company's
Board of Directors.

(b)    Note Payable to Lenders

In connection with the purchase by the Lenders of the note payable
to the bank, the Lenders lent an additional $3,200,000 so as to
increase the outstanding principal amount of the note to
$6,000,000. The terms of the note payable were amended as follows:
(i) the maturity was extended to November 30, 2003; (ii) the
interest rate was decreased to 8%; (iii) interest is payable
monthly in arrears, with the principal due in full at maturity of
the loan; (iv) the note payable is convertible, at the Lenders'
option, in whole or in part, into shares of common stock at a
conversion price equal to $2.40 per share; (v) the note includes a
minimum liquidity, as defined covenant of $2,000,000; and (vi) the
note payable may not be prepaid, in whole or in part, at any time
prior to December 1, 2000. On March 30, 1999, the Company received
a waiver for noncompliance with the minimum tangible net worth
covenant effective as of December 31, 1998 and March 31, 1999. On
April 15, 1999, the Company also received a waiver for
non-compliance with the minimum liquidity covenant effective as of
April 15, 1999. The Company has classified the outstanding balance
of $6,000,000 at December 31, 1998 as a current liability in the
accompanying consolidated balance sheet as it does not currently
have the financing to remain in compliance with the financial
covenants. In connection with the purchase of the note payable,
Forum is entitled to receive $400,000 as a fee, which Forum has
agreed to reinvest by purchasing common stock or preferred stock,
both with attached warrants. The Company has recorded the $400,000
as a deferred financing cost, which will be amortized to interest
expense over the term of the note and an accrued expense for the
issuance of common stock or preferred stock, both with attached
warrants, which will occur in 1999. In addition, Forum is entitled
to receive warrants to purchase $400,000 of shares of common stock
of the Company at the per share valuation of the next financing,
or $3.00 per share if the financing is not completed by May 1,
1999. The Company determined the value of the warrants to be
$85,433, by using the Black-Scholes option pricing model. The
Company has recorded this $85,433 as a deferred financing cost,
which will be amortized to interest expense over the term of the
note. (See Note 22 for additional information through February 1,
2000.)

(c)    Note Payable to Landlord

In December 1994, the Company issued a $750,000 promissory note to
its landlord to fund specific construction costs associated with
the development of its manufacturing plant in Milford,
Massachusetts. The promissory note bears interest at 13% per annum
and is to be paid in equal monthly installments of principal and
interest over the remainder of the 10-year lease term.

(d)    Capital Lease Obligations

The Company had entered into various capital leases for equipment.
During 1998, the Company settled its capital lease obligations in
full through the issuance of common stock and warrants (see Note
15(c)).

(e)    9% Convertible Subordinated Notes Payable

On April 2, 1997, the Company issued $50,000,000 of the 9% Notes.
Under the terms of the 9% Notes, the Company must make semiannual
interest payments on the outstanding

F-13

principal balance through the maturity date of April 1, 2004. If
the 9% Notes are converted prior to April 1, 2000, the noteholders
are entitled to receive accrued interest from the date of the most
recent interest payment through the conversion date. The 9% Notes
are convertible at any time prior to the maturity date at a
conversion price equal to $35.0625, subject to adjustment under
certain circumstances, as defined.

Beginning April 1, 2000, the Company may redeem the 9% Notes at
its option for a 4.5% premium over the original issuance price
provided that from April 1, 2000 to March 31, 2001, the 9% Notes
may not be redeemed unless the closing price of the common stock
equals or exceeds 150% of the conversion price for a period of at
least 20 out of 30 consecutive trading days and the 9% Notes are
redeemed within 60 days after such trading period. The premium
decreases by 1.5% each year through March 31, 2003. Upon a change
of control of the Company, as defined, the Company will be
required to offer to repurchase the 9% Notes at 150% of the
original issuance price.

On February 6, 1998, the Company commenced an exchange offer to
the holders of the 9% Notes to exchange the 9% Notes for Series A
convertible preferred stock and warrants. On May 5, 1998,
noteholders holding $48,694,000 of principal and $2,361,850 of
accrued interest tendered such principal and accrued interest to
the Company for 510,505 shares of Series A convertible preferred
stock and warrants to purchase 3,002,958 shares of common stock
with an exercise price of $4.25 per share. In accordance with SFAS
No. 15, Accounting by Debtors and Creditors for Troubled Debt
Restructurings, the Company recorded an extraordinary gain of
$8,876,685 related to the exchange. The extraordinary gain
represents the difference between the carrying value of the 9%
Notes plus accrued interest, less $2,249,173 of deferred financing
costs written off, and the fair value of the Series A convertible
preferred stock, as determined by the per share sales price of
Series A convertible preferred stock sold in the 1998 Unit
Financing (see Note 15(c)), and warrants to purchase common stock
issued by the Company.

(8)    G.D. SEARLE & CO. AGREEMENT

In January 1996, the Company and G.D. Searle & Co. (Searle) entered into
a collaboration relating to research and development of therapeutic
antisense compounds. According to the collaboration agreement, as
modified in April 1998, targets can be selected from those in the fields
of cancer, cardiovascular disease and inflammation/immunomodulation (the
Searle Field).

Pursuant to the collaboration, the parties are conducting research and
development relating to a compound directed at MDM2. In this project,
Searle is funding certain research and development efforts by the
Company, and both Searle and the Company have committed certain of its
own personnel to the collaboration. The initial phase of research and
development activities will be conducted through the earlier of (i) the
achievement of certain milestones, and (ii) January 31, 2000, subject to
early termination by Searle. The parties may extend the initial
collaboration by mutual agreement, including agreement as to additional
research funding by Searle. Searle has advised the Company that it
intends to inform the Company by February 29, 2000 whether it intends to
extend this collaboration.

In addition, under the collaboration, Searle has the right to designate
up to six additional molecular targets in the Searle Field (the
Additional Targets) on terms substantially consistent with the terms of
the collaboration applicable to the initial molecular target. This right
is exercisable by Searle with respect to each of the Additional Targets
upon the payment by Searle of certain research payments (beyond the
project-specific payments relating to the particular Additional Target)
and the purchase of additional common stock from the Company by Searle
(at the then fair market value). The aggregate amount to be paid by
Searle for such research payments and equity investment in order to
designate each of the Additional Targets is $10,000,000 per Additional
Target. In the event that Searle designates all of the Additional
Targets, the aggregate amount to be paid by Searle for research payments
will be $24,000,000, and the aggregate amount to be paid by

F-14

Searle in equity investment will be $36,000,000. If Searle has not
designated all of the Additional Targets by the time the initial
molecular target reaches a certain stage of preclinical development,
Searle will be required to purchase an additional $10,000,000 of common
stock (at the then fair market value) in order to maintain its right to
designate any of the Additional Targets. The payment for any such common
stock will be creditable against the equity investment portion of the
payments to be made by Searle with respect to the designation of any of
the Additional Targets that Searle has not yet designated.

Searle has exclusive rights to commercialize any products resulting from
the collaboration. If Searle elects to commercialize a product, Searle
will fund and perform preclinical tests and clinical trials of the
product candidate and will be responsible for regulatory approvals for
and marketing of the product. The Company has agreed to perform research
and development work exclusively with Searle. In addition, for each
product candidate, the Company will be entitled to milestone payments
from Searle totaling up to an aggregate of $10,000,000 upon the
achievement of certain development benchmarks. The Company also will be
entitled to royalties from net sales of products resulting from the
collaboration. Subject to satisfying certain conditions relating to its
manufacturing capacities and capabilities, the Company will retain
manufacturing rights, and Searle will be required to purchase its
requirements of products from the Company on an exclusive basis at
specified prices. Upon a change in control of the Company, Searle would
have the right to terminate the Company's manufacturing rights, although
the royalty payable would be increased in such event.

In the event that Searle designates all of the Additional Targets or if
Hybridon fails to satisfy certain requirements relating to its
manufacturing capacities and capabilities, Searle will have the right to
require Hybridon to form a joint venture with Searle, as defined. The
Company and Searle would each own 50% of the joint venture, although
Searle's ownership interest in the joint venture would increase based
upon a formula to up to a maximum of 75% if the joint venture is
established in certain instances relating to the Company's failure to
satisfy certain requirements relating to its manufacturing capacities and
capabilities.

During 1996, 1997 and 1998, the Company earned $400,000, $600,000 and
$600,000, respectively, in research and development revenues from Searle.
Under the collaboration, Searle also purchased 200,000 shares of common
stock in the Company at the offering price of $50.00 per share.

(9)    F. HOFFMANN-LA ROCHE LTD. (ROCHE) COLLABORATION

In December 1992, the Company and Roche entered into a collaboration
involving the application of the Company's antisense oligonucleotide
chemistry to develop compounds for the treatment of hepatitis B,
hepatitis C and human papilloma virus. On September 3, 1997, Roche
notified the Company that it had decided not to pursue further
collaboration with the Company and was terminating the collaboration
effective February 28, 1998.

The Company has recorded $1,019,389 and $345,000 of research and
development revenue related to this collaboration in 1996 and 1997,
respectively. Due to the termination of the collaboration, as discussed
above, the Company recognized no revenue with respect to this
collaboration in 1998.

(10)   MEDTRONIC, INC. COLLABORATIVE STUDY AGREEMENT

In May 1994, the Company and Medtronic, Inc. (Medtronic) entered into a
collaborative study agreement (the Medtronic Agreement) involving the
development of antisense compounds for the treatment of Alzheimer's
disease and a drug delivery system to deliver such compounds into the
central nervous system. The agreement provides that the Company is
responsible for the development of, and hold all rights to, any drug
developed pursuant to this collaboration, and Medtronic is responsible
for the development of, and hold all rights to, any delivery system

F-15

developed pursuant to this collaboration. The parties may extend this
collaboration by mutual agreement to other neurodegenerative disease
targets. The Company is not currently conducting any activities under
this collaboration.

(11)   LICENSING AGREEMENT

The Company has entered into a licensing agreement with the Worcester
Foundation for Biomedical Research, Inc., which has merged with the
University of Massachusetts Medical Center, under which the Company has
received exclusive licenses to certain patents and patent applications.
The Company is required to make royalty payments based on future sales of
products employing the technology or falling under claims of a patent, as
well as a specified percentage of sublicense income received related to
the licensed technology. Additionally, the Company is required to pay an
annual maintenance fee through the life of the patents.

(12)   PHARMACIA BIOTECH, INC. COLLABORATION

In December 1994, the Company and Pharmacia Biotech, Inc. (Pharmacia)
entered into a collaboration involving the design and development of a
large-scale oligonucleotide synthesis machine. Following completion of
the machine in December 1996, the collaboration expired, and Pharmacia
retained the right to sell the machine to third parties, subject to an
obligation to pay the Company royalties on such third-party sales. During
1996 and 1997, the Company received $62,321 and $48,000, respectively, of
royalty income related to such third-party sales. The Company recognized
no royalty income related to this collaboration for 1998.

(13)   PERKIN-ELMER CORPORATION SALES AND SUPPLY AGREEMENT

In September 1996, the Company and the Applied Biosystems Division of
Perkin-Elmer Corporation (Perkin-Elmer) signed a four-year sales and
supply agreement under which Perkin-Elmer agreed to refer potential
customers to HSP for the manufacture of custom oligonucleotides and the
Company agreed that amidites for the manufacture of these
oligonucleotides would be purchased from Perkin-Elmer and a percentage of
the sales price will be paid to Perkin-Elmer. In addition, Perkin-Elmer
licensed to the Company its oligonucleotide synthesis patents.

(14) INVESTMENT IN METHYLGENE, INC.

In January 1996, the Company and three Canadian institutional investors
formed a Quebec company, MethylGene, Inc. (MethylGene) to develop and
market certain compounds and procedures to be agreed upon by the Company
and MethylGene.

The Company has granted to MethylGene exclusive worldwide licenses and
sublicenses in respect of certain technology relating to the MethylGene
fields. These fields, as amended, are defined as (i) antisense compounds
to inhibit DNA methyltransferase for the treatment of any disease; (ii)
other methods of inhibiting DNA methyltransferase for the treatment of
any disease; and (iii) antisense compounds to inhibit up to two
additional molecular targets for the treatment of cancers, to be agreed
upon by the Company and MethylGene. In addition, the Company and
MethylGene have entered into a supply agreement pursuant to which
MethylGene is obligated to purchase from the Company all required
formulated bulk oligonucleotides at specified transfer prices.

The Company acquired a 49% interest in MethylGene for approximately
$734,000, and the Canadian investors acquired a 51% interest in
MethylGene for a total of approximately $5,500,000. The institutional
investors have the right to exchange all (but not less than all) of their
shares of stock in MethylGene for an aggregate of 100,000 shares of
Hybridon common stock (subject to adjustment for stock splits, stock
dividends and the like). This option is exercisable only during a

F-16

90-day period commencing on the earlier of the date five years after the
closing of the institutional investors' investment in MethylGene or the
date on which MethylGene ceases operations. This option terminates sooner
if MethylGene raises certain additional amounts of equity or debt
financing or if MethylGene enters into a corporate collaboration that
meets certain requirements. During 1998, MethylGene raised additional
proceeds from outside investors that decreased the Company's interest to
30%. The Company is accounting for its investment in MethylGene under the
equity method and, due to the existence of the investors exchange rights,
the Company has recorded, up to its original investment, 100% of
MethylGene's losses in the accompanying consolidated statements of
operations.

In May 1998, this agreement was amended to grant MethylGene a
non-exclusive right to use any and all antisense chemistries discovered
by the Company or any of its affiliates for a period commencing on May 5,
1998 and ending on the earlier of (i) the effective date of termination
by MethylGene of its contract for development services to be provided by
the Company; (ii) May 5, 1999, unless MethylGene exercises its option to
continue contracting for development services provided by the Company; or
(iii) May 5, 2000. As additional consideration for this nonexclusive
right, MethylGene is required to pay the Company certain milestone
amounts, as defined, and transferred 300,000 shares of MethylGene's Class
B shares to the Company. The Company has placed no value on these shares.
During 1996, 1997, 1998 and the nine months ended September 30, 1998 and
1999, the Company recognized $49,565, $101,894, $1,685,932, $1,552,381
and $920,814 respectively, of product and service revenue related to this
agreement.

(15)   STOCKHOLDERS' EQUITY (DEFICIT)

(a)    Common Stock

The Company has 100,000,000 authorized shares of common stock,
$.001 par value, of which 15,304,825 shares were issued and
outstanding at December 31, 1998.

(b)    Initial Public Offering (IPO)

On February 2, 1996, the Company completed its IPO of 1,150,000
shares of common stock at $50.00 per share. The sale of common
stock resulted in net proceeds to the Company of $52,231,244 after
deducting expenses related to the offering.

(c)    1998 Unit Financing

On May 5, 1998, the Company completed a private offering of equity
securities raising total gross proceeds of $26,681,164 from the
issuance of 9,597,476 shares of common stock, 114,285 shares of
Series A convertible preferred stock and warrants to purchase
3,329,486 shares of common stock at $2.40 per share. The gross
proceeds include the conversion of $5,934,558 of accounts payable,
capital lease obligations and other obligations into common stock.
The Company incurred $1,636,137 of cash expenses related to the
private offering and issued 597,699 shares of common stock and
warrants to purchase 1,720,825 shares of common stock at $2.40 per
share to the placement agents. The compensation received by
Pillar, a company affiliated with certain directors of the
Company, with respect to the offshore component of the private
offering (Offshore Offering) consisted of (i) 9% of gross proceeds
of such Offshore Offerings and (ii) a nonaccountable expense
allowance equal to 4% of gross proceeds of such Offshore Offering.
Pillar received $1,636,137 and warrants to purchase 1,111,630
shares of common stock at $2.40 per share.

In addition, Pillar is entitled to receive 300,000 shares of
common stock in connection with its efforts in assisting the
Company in restructuring its balance sheet. The Company has
recorded $600,000 of general and administrative expense in the
accompanying consolidated

F-17

statement of operations during 1998, which represents the value of
this common stock on May 5, 1998 with an offsetting amount to
accrued expenses for the shares to be issued. These shares will be
issued in 1999.

(d)    Units Issued to Primedica Corporation

In connection with the unit financing (see Note 15(c)) the Company
issued 250,000 shares of common stock and 62,500 warrants to
purchase common stock to Primedica Corporation (Primedica) for
future services to be provided. The services shall commence upon
the Company's request after (i) the Company's securities are
listed on a nationally recognized exchange, and (ii) the average
closing price of the Company's common stock is at least $2.00 per
share for the twenty-day trading period preceding the contract
commencement date. In the event that the Company does not use
these services as a result of the failure to meet the contract
conditions, Primedica shall forfeit to the Company all or part of
the common stock and warrants held by Primedica. The Company has
recorded these shares as issued and outstanding at December 31,
1998 at par value. The Company will record the value of these
services as the services are rendered.

(e)    Stock Split

On December 10, 1997, the Board of Directors declared a
one-for-five reverse split of its common stock. Share quantities
and related per share amounts have been retroactively restated to
reflect the reverse stock split.

(f)    Warrants

The Company has the following warrants outstanding and exercisable
for the purchase of common stock at December 31, 1998:

Expiration Date                      Outstanding    Exercise Price     Exercisable    Exercise Price
Warrants         per Share        Warrants         per Share
---------------- ---------------- ---------------- ----------------

February 4, 1999-October 25, 2000       551,201        $ 50.00            551,201          $50.00
February 28, 2000                        20,000          37.50             20,000           37.50
December 31, 2001                        13,000          34.49             13,000           34.49
May 4, 2003                           8,641,503           2.40-          4,378,044           2.40
------------       ---------         ----------         -------
4.25
9,225,704                          4,962,245
============                          =========
Weighted  average  exercise price                        $5.48                              $7.91
per share                                                =====                              =====

Five-year warrants to purchase 368,620 shares of common stock at
$50.00 per share were issued in 1994 and 1995 as a component of
the compensation for services of several placement agents of the
Company's convertible preferred stock. Of these warrants, 304,335
were issued to a company that is controlled by two directors of
the Company (see Note 16(b)). The remaining 64,285 warrants were
issued to various other companies that acted as placement agents.
See Note 15(c) for information relating to warrants issued to
placement agents in connection with the 1998 Unit Financing.

As consideration of the agreements made by Forum consenting to the
Company's 1998 private placements and waiving certain obligations
of the Company to Forum, the Company agreed to amend the warrant
to purchase 71,301 shares of common stock at an exercise price

F-18

of $35.06 per share, issued to Forum in connection with 9% notes
so that the exercise price will be equal to $4.25 per share, and
the number of shares of common stock purchasable upon exercise
thereof will be increased to 588,235, in each case subject to
adjustment; provided, however, that such warrant will also be
amended to provide that such warrant may not be exercised until
May 5, 1999 and the transactions contemplated by such private
placements and by the exchange offer will not trigger any
anti-dilution adjustments to the exercise price thereof or the
number of shares of common stock subject thereto.

(g)    Stock Options

In 1990 and 1995, the Company established the 1990 Stock Option
Plan (the 1990 Option Plan) and the 1995 Stock Option Plan (the
1995 Option Plan), respectively, which provide for the grant of
incentive stock options and nonqualified stock options. Options
granted under these plans vest over various periods and expire no
later than 10 years from the date of grant. However, under the
1990 Option Plan, in the event of a change in control (as defined
in the 1990 Plan), the exercise dates of all options then
outstanding shall be accelerated in full and any restrictions on
exercising outstanding options issued pursuant to the 1990 Option
Plan shall terminate. In October 1995, the Company terminated the
issuance of additional options under the 1990 Option Plan. As of
December 31, 1998, options to purchase a total of 525,638 shares
of common stock remained outstanding under the 1990 Option Plan.

A total of 700,000 shares of common stock may be issued upon the
exercise of options granted under the 1995 Option Plan. The
maximum number of shares with respect to which options may be
granted to any employee under the 1995 Option Plan shall not
exceed 500,000 shares of common stock during any calendar year.
The Compensation Committee of the Board of Directors has the
authority to select the employees to whom options are granted and
determine the terms of each option, including (i) the number of
shares of common stock subject to the option; (ii) when the option
becomes exercisable; (iii) the option exercise price, which, in
the case of incentive stock options, must be at least 100% (110%
in the case of incentive stock options granted to a stockholder
owning in excess of 10% of the Company's common stock) of the fair
market value of the common stock as of the date of grant; and (iv)
the duration of the option (which, in the case of incentive stock
options, may not exceed 10 years). As of December 31, 1998,
options to purchase a total of 550,534 shares of common stock
remained outstanding under the 1995 Option Plan.

In October 1995, the Company adopted the 1995 Director Stock
Option Plan (the Director Plan). A total of 50,000 shares of
common stock may be issued upon the exercise of options granted
under the Director Plan. Under the terms of the Director Plan,
options to purchase 1,000 shares of common stock were granted to
eligible directors upon the closing of the Company's initial
public offering at the fair market value of the common stock on
the date of the closing. Thereafter, options to purchase 1,000
shares of common stock will be granted to each eligible director
on May 1 of each year commencing in 1997. All options will vest on
the first anniversary of the date of grant or, in the case of
annual options, on April 30 of each year with respect to options
granted in the previous year. As of December 31, 1998, options to
purchase a total of 21,000 shares of common stock remained
outstanding under the Director Plan.

In May 1997, the Company adopted the 1997 Stock Option Plan (the
1997 Option Plan) and has reserved and may issue up to 4,500,000
shares for the grant of incentive and nonqualified stock options.
The maximum number of shares with respect to which options may be
granted to any employee under the 1997 Option Plan shall not
exceed 500,000 shares of common stock during any calendar year.
The Compensation Committee of the Board of Directors has the
authority to select the employees to whom options are granted and
determine the terms of each option, including (i) the number of
shares of common stock subject to the option; (ii)

F-19

when the option becomes exercisable; (iii) the option exercise
price, which, in the case of incentive stock options, must be at
least 100% (110% in the case of incentive stock) of the fair
market value of the common stock as of the date of grant; and (iv)
the duration of the option (which, in the case of incentive stock
options, may not exceed ten years). As of December 31, 1998,
options to purchase a total of 2,363,560 shares of common stock
remained outstanding under the 1997 Option Plan. See Note 22(g).

Stock option activity for the three years ended December 31, 1998
is summarized as follows:

Number         Exercise Price         Weighted
of Shares          per Share        Average Price
per Share
---------------   ---------------   ---------------

Outstanding, December 31, 1995                 738,208          $.01-$50.00            $29.15
Granted                                       476,020          25.00-65.60             49.55
Exercised                                    (57,740)            .01-37.50             18.85
Terminated                                   (20,100)          25.00-57.85             40.20
-------
Outstanding, December 31, 1996               1,136,388           1.25-65.60             38.05
Granted                                       315,675          27.50-32.50             30.75
Exercised                                    (25,005)           1.25-40.00             12.60
Terminated                                  (236,561)           2.50-65.60             40.35
--------
Outstanding, December 31, 1997               1,190,497           1.25-65.60             36.18
Granted                                     2,513,000            2.00-3.13              2.00
Terminated                                  (242,765)           2.50-57.85             37.79
--------
Outstanding, December 31, 1998               3,460,732         $1.25-$65.60            $11.25
=========         ============            ======
Exercisable, December 31, 1996                 622,930         $1.25-$65.60            $32.55
=======         ============            ======
Exercisable, December 31, 1997                 740,780         $1.25-$65.50            $34.40
=======         ============            ======
Exercisable, December 31, 1998               1,650,021         $1.25-$65.60            $17.13
=========         ============            ======

Options Outstanding              Options Exercisable
Range of Exercise         Number         Weighted        Weighted         Number        Weighted
Prices           Outstanding       Average         Average       Outstanding       Average
Remaining        Exercise                       Exercise
Contractual      Price per                       Price per
Life           Share                           Share
----------------------- --------------- --------------- --------------- --------------- --------------

$1.25           10,000           3.10         $   1.25           10,000        $ 1.25
2.00-2.37        2,505,000           9.56             2.00          901,562          2.00
2.44-3.13           18,800           6.03             2.61           10,800          2.50
4.25-5.00            1,200           3.75             5.00            1,200          3.75
17.50-2.00          197,330           3.54            23.21          191,331         23.15
27.50-31.66          168,974           7.45            30.50           76,017         30.28
35.00-36.25           30,000           6.73            35.71           30,000         35.71
37.50          316,048           4.72            37.50          282,583         37.50
38.13-43.75           47,900           7.81            40.64           24,648         40.73
50.00           17,700           6.35            50.00           11,700         50.00
57.85-65.60          147,780           6.08            58.22          110,180         58.34
-------                                          -------
3,460,732                          $11.25        1,650,021        $17.13
=========                          ======        =========        ======

In October 1995, the Financial Accounting Standards Board issued
SFAS No. 123, Accounting for Stock-Based Compensation. SFAS No.
123 requires the measurement of the fair value of stock options or
warrants granted to employees to be included in the statement of

F-20

operations or disclosed in the notes to financial statements. The
Company has determined that it will continue to account for
stock-based compensation for employees under Accounting Principles
Board Opinion No. 25 and elect the disclosure-only alternative
under SFAS No. 123. In 1996, 1997 and 1998, the Company recorded
$1,967,116, $205,978 and $109,734, respectively, of deferred
compensation related to grants to nonemployees, net of
terminations. Deferred compensation will be amortized over the
vesting period of the options. The Company has recorded
compensation expense of $763,190, $316,067 and $246,444 in 1996,
1997 and 1998, respectively, related to these grants to
nonemployees.

The Company has computed the pro forma disclosures require by SFAS
No. 123 for all stock options granted after January 1, 1995 using
the Black-Scholes option pricing model. The assumptions used for
the three years ended December 31, 1998 are as follows:

1996         1997          1998
----         ----          ----
Risk free interest rate      6.14%        6.22%          5.15%
Expected dividend yield        -            -              -
Expected lives              6 years      6 years        6 years
Expected volatility           60%          60%            60%

The Black-Scholes option pricing model was developed for use in
estimating the fair value of traded options which have no vesting
restrictions and are fully transferable. In addition, option
pricing models require the input of highly subjective assumptions
including expected stock price volatility. Because the Company's
employee stock options have characteristics significantly
different from those of traded options, and because changes in the
subjective input assumptions can materially affect the fair value
estimate, in management's opinion, the existing models do not
necessarily provide a reliable single measure of the fair value of
its employee stock options.

The effect of applying SFAS No. 123 for the three years ended
December 31, 1998 would be as follows:

1996               1997                1998
----               ----                ----

Net loss applicable to common
stockholders-
As reported                             $    (46,852,600)  $    (69,461,326)  $    (19,792,736)
================   ================   ================
Pro forma                               $    (52,890,455)  $    (73,402,170)  $    (23,131,304)
================   ================   ================
Basic and Diluted net loss per
common shares-
As reported                                 $(10.24)           $(13.76)            $(1.67)
=======            ========            =======
Pro forma                                   $(11.56)           $(14.54)            $(1.95)
=======            ========            ======

(h)    Employee Stock Purchase Plan

In October 1995, the Company adopted the 1995 Employee Stock
Purchase Plan (the Purchase Plan), under which up to 100,000
shares of common stock may be issued to participating employees of
the Company, as defined, or its subsidiaries.

On the first day of a designated payroll deduction period (the
Offering Period), the Company will grant to each eligible employee
who has elected to participate in the Purchase Plan an option to
purchase shares of common stock as follows: the employee may
authorize an amount (a whole percentage from 1% to 10% of such
employee's regular pay) to be deducted

F-21

by the Company from such pay during the Offering Period. On the
last day of the Offering Period, the employee is deemed to have
exercised the option, at the option exercise price, to the extent
of accumulated payroll deductions. Under the terms of the Purchase
Plan, the option price is an amount equal to 85% of the fair
market value per share of the common stock on either the first day
or the last day of the Offering Period, whichever is lower. In no
event may an employee purchase in any one Offering Period a number
of shares which is more than 15% of the employee's annualized base
pay divided by 85% of the market value of a share of common stock
on the commencement date of the Offering Period. The Compensation
Committee may, in its discretion, choose an Offering Period of 12
months or less for each of the Offerings and choose a different
Offering Period for each Offering. No shares have been issued
under the Plan.

(i)    Preferred Stock

The restated Certificate of Incorporation of the Company permits
its Board of Directors to issue up to 5,000,000 shares of
preferred stock, par value $.01 per share (the Preferred Stock),
in one or more series, to designate the number of shares
constituting such series, and fix by resolution, the powers,
privileges, preferences and relative, optional or special rights
thereof, including liquidation preferences and dividends, and
conversion and redemption rights of each such series. During 1998,
the Company designated 1,500,000 shares as Series A convertible
preferred stock.

(j)    Series A Convertible Preferred Stock

The rights and preferences of the Series A convertible preferred
stock are as follows:

Dividends

The holders of the Series A convertible preferred stock, as of
March 15 or September 15, are entitled to receive dividends
payable at the rate of 6.5% per annum, payable semi-annually in
arrears. Such dividends shall accrue from the date of issuance of
such share and shall be paid semi-annually on April 1 and October
1 of each year. Such dividends shall be paid, at the election of
the Company, either in cash or additional duly authorized, fully
paid and non assessable shares of Series A convertible preferred
stock. In calculating the number of shares of Series A convertible
preferred stock to be paid with respect to each dividend, the
Series A convertible preferred stock shall be valued at $100.00
per share. During 1998, the Company recorded a total accretion of
$2,689,048 for the dividend on Series A preferred stock and issued
16,470 shares of Series A convertible preferred stock as a
dividend.

Liquidation

In the event of a liquidation, dissolution or winding up of the
Company, whether voluntary or involuntary, after payment or
provision for payment of debts and other liabilities of the
Company, the holder of the Series A convertible preferred stock
then outstanding shall be entitled to be paid out of the assets of
the Company available for distribution to its stockholders, an
amount equal to $100.00 per share plus all accrued but unpaid
dividends. If the assets to be distributed to the holders of the
Series A convertible preferred stock shall be insufficient to
permit the payment of the full preferential amounts, then the
assets of the Company shall be distributed ratably to the holders
of the Series A convertible preferred stock on the basis of the
number of shares of Series A convertible preferred stock held. All
shares of Series A convertible preferred stock shall rank as to
payment upon the occurrence of any liquidation event senior to the
common stock.

Conversion

F-22

Commencing after May 6, 1999, but not prior thereto, the shares of
Series A convertible preferred stock shall be convertible, in
whole or in part, at the option of the holder into fully paid and
nonassessable shares of common stock at $4.25 per share, subject
to adjustment as defined.

Mandatory Conversion

At any time after May 6, 1998, the Company may at its option,
cause the Series A convertible preferred stock to be converted in
whole or in part, on a pro rata basis, into fully paid and
nonassessable shares of common stock using a conversion price
equal to $4.00 if the closing bid price, as defined, of the common
stock shall have equaled or exceeded 250% of the conversion price,
$4.25, subject to adjustment as defined, for at least 20 trading
days in any 30 consecutive trading day period ending three days
prior to the date of notice of conversion (such event, the Market
Trigger).

At any time after April 1, 2000, the Company, at its option, may
redeem the Series A convertible preferred stock for cash equal to
$100.00 per share plus all accrued and unpaid dividends at such
time, if the Market Trigger has occurred in the period ending
three days prior to the date of notice of redemption.

(16)   COMMITMENTS AND CONTINGENCIES

(a)    Facilities

The Company leases its facility in Milford, Massachusetts, under a
lease which has a 10-year term, which commenced on July 1, 1994,
with certain extension options.

On February 4, 1994, the Company entered into the Cambridge Lease
with a partnership that is affiliated with certain directors of
the Company. As compensation for arranging this lease, the Company
issued Pillar Limited five-year warrants for the purchase of
100,000 shares of the Company's common stock at an exercise price
of $50.00 per share. These warrants expired subsequent to December
31, 1998. The Company vacated the Cambridge, Massachusetts
facility in June 1998 and moved its corporate facilities to
Milford, Massachusetts (see Note 3).

Future approximate minimum rent payments as of December 31, 1998,
under existing lease agreements through 2007, net of sublease
agreements are as follows:

December 31,           Amount      
------------           ------      
1999               $     614,000   
2000                     784,000   
2001                   1,213,000   
2002                   1,209,000   
2003                   1,213,000   
Thereafter             2,338,000   
-------------   

$   7,371,000   

During 1996, 1997 and 1998, facility rent expense net of sublease
revenue was approximately $2,352,000, $4,613,000 and $3,871,000,
respectively.

(b)    Related-Party Agreements with Affiliates of Stockholders and 
Directors

F-23

The Company has entered into consulting agreements, stock
placement agreements and an advisory agreement with several
companies that are controlled by two shareholders and directors of
the Company including Forum, S.A. Pillar Investment N.V. (Pillar
Investment), Pillar S.A. (formerly Commerce Consult S.A.) and
Pillar Investment Limited (formerly Ash Properties Limited)
(Pillar Limited). During 1996, 1997 and 1998, the Company had
expensed $1,106,000, $998,000 and $1,300,000, respectively, under
consulting and advisory agreements with related parties.

(c)    Other Research and Development Agreements

The Company has entered into consulting and research agreements
with the universities, research and testing organizations and
individuals, under which consulting and research support is
provided to the Company. These agreements are for varying terms
and provide for certain minimum annual or per diem fees plus
reimbursable expenses to be paid during the contract periods.
Future minimum fees payable under these contracts as of December
31, 1998 are approximately as follows:

December 31,             Amount
------------             ------
1999             $     582,000
2000                   392,000
2001                   279,000
-------------
$   1,253,000

Total fees and expenses under these contracts were approximately
$7,171,000, $9,372,000 and $2,011,000 during 1996, 1997 and 1998,
respectively.

(d)    Employment Agreements

The Company has entered into employment agreements with its
executive officers which provide for, among other things, each
officer's annual salary, cash bonus, fringe benefits, and vacation
and severance arrangements. Under the agreements, the officers are
generally entitled to receive severance payments of two to three
years' base salary.

(e)    Contingencies

From time to time, the Company may be exposed to various types of
litigation. The Company is not engaged in any legal proceedings
that are expected, individually or in the aggregate, to have a
material adverse effect on the Company's financial condition or
results of operations.

(17)   INCOME TAXES

The Company applies SFAS No. 109, Accounting for Income Taxes. At
December 31, 1998, the Company had net operating loss and tax credit
carryforwards for federal income tax purposes of approximately
$219,993,000 and $3,936,000, respectively, available to reduce federal
taxable income and federal income taxes, respectively. The Tax Reform Act
of 1986 (the Act), enacted in October 1986, limits the amount of net
operating loss and credit carryforwards that companies may utilize in any
one year in the event of cumulative changes in ownership over a
three-year period in excess of 50%. The Company has completed several
financings since the effective date of the Act, which, as of December 31,
1998, have resulted in ownership changes in excess of 50%, as defined
under the Act and which will limit the Company's ability to utilize its
net operating loss

F-24

carryforwards. Ownership changes in future periods may place additional
limits on the Company's ability to utilize net operating loss and tax
credit carryforwards.

The federal net operating loss carryforwards and tax credit carryforwards
expire approximately as follows:

F-25

Net
Operating Loss         Tax Credit
Expiration Date          Carryforwards       Carryforwards

December 31,
2005                   $   666,000        $   15,000
2006                     3,040,000            88,000
2007                     7,897,000           278,000
2008                    18,300,000           627,000
2009                    25,670,000           689,000
2010                    36,134,000           496,000
2011                    44,947,000           493,000
2012                    60,087,000           750,000
2018                    23,252,000           500,000
----------           -------
$219,993,000        $3,936,000

At December 31, 1997 and 1998, the components of the deferred tax assets
are approximately as follows:

1997                 1998
----                 ----

Operating loss carryforwards       $ 78,696,000        $ 87,997,000
Temporary differences                 5,137,000           2,677,000
Tax credit carryforwards              3,436,000           3,936,000
---------           ---------

87,269,000          94,610,000

Valuation allowance                (87,269,000)         (94,610,000)
-----------          -----------

$        --         $         --
============        ============

A valuation allowance has been provided, as it is more likely than not
the Company will not realize the deferred tax asset. The net change in
the total valuation allowance during 1998 was an increase of
approximately $7,341,000.

(18)   EMPLOYEE BENEFIT PLAN

On October 10, 1991, the Company adopted an employee benefit plan under
Section 401(k) of the Internal Revenue Code. The plan allows employees to
make contributions up to a specified percentage of their compensation.
Under the plan, the Company may, but is not obligated to, match a portion
of the employees' contributions up to a defined maximum. The Company is
currently matching 50% of employee contributions to the plan, up to 6% of
the employee's annual base salary, and charged to operations
approximately $224,000, $253,000 and $253,000 during 1996, 1997 and 1998,
respectively.

(19)   SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION

Supplemental disclosure of cash flow information for the three years in
the period ended December 31, 1998 are as follows:

F-26

1996           1997             1998
----           ----             ----

Cash paid during the period for interest                      $   124,052   $    3,264,596   $ 1,666,127
===========   ==============   ===========

Purchase of property and equipment under capital leases       $ 1,722,333   $    2,374,502   $        --
===========   ==============   ===========

Conversion of preferred stock into common stock               $   159,822   $           --   $        --
===========   ==============   ===========

Deferred compensation related to grants of stock options to   $ 1,967,116   $      205,978   $   109,734
nonemployees, net of terminations                           ===========   ==============   ===========

Issuance of Series A convertible preferred stock and          $        --   $           --   $51,055,850
attached warrants in exchange for conversion of 9%          ===========   ==============   ===========
convertible subordinated notes payable and accrued
interest

Accretion of Series A convertible preferred stock dividends   $        --   $           --   $ 2,689,048
===========   ==============   ===========

Issuance of common stock and attached warrants in exchange    $        --   $           --   $ 4,800,000
for conversion of convertible promissory notes payable      ===========   ==============   ===========

Issuance of common stock and attached warrants in exchange    $        --   $           --   $ 5,934,558
for conversion of accounts payable and other obligations    ===========   ==============   ===========

(20)     RESTATEMENT

In March 1999, the Company restated its June 30, 1998 and September 30,
1998 financial statements to reflect the accretion on the Series A
convertible preferred stock, and record $600,000 of general and
administrative expense for the 300,000 shares of common stock that
Pillar is entitled to receive in connection with its efforts in
assisting the Company in restructuring its balance sheet.

(21)     ORIGENIX TECHNOLOGIES, INC.

In January 1999, the Company and certain institutional investors formed
a Montreal company, OriGenix Technologies Inc. (OriGenix), to develop
and market drugs for the treatment of infectious diseases.

The Company received a 49% interest in OriGenix in consideration of
certain research and development efforts previously undertaken by the
Company which were made available to OriGenix. The Company has also
licensed certain antisense compounds and other technology to OriGenix.
If certain conditions are satisfied by OriGenix, the institutional
investors are committed to make an additional investment, at which time
the Company's ownership interest in OriGenix will be reduced to 40%.
The institutional investors acquired a 51% interest in OriGenix for a
total of approximately $4.0 million. The Company will account for its
investment in OriGenix under the equity method.

(22)     INTERIM PERIOD AND SUBSEQUENT EVENTS (Unaudited)

(a)   Unaudited Interim Financial Statements

The accompanying consolidated balance sheet as of September 30,
1999, and the consolidated statements of operations,
stockholders' equity (deficit) and cash flows for the three and
nine months ended September 30, 1998 and 1999 are unaudited, but,
in the opinion of management, have been prepared on a basis
substantially consistent with audited financial statements and
include all adjustments, consisting of only normal recurring
adjustments, necessary for a fair presentation of the results of
these interim periods. The

F-27

results for the period ended September 30, 1999 are not
necessarily indicative of results to be expected for the full
fiscal year.

In October 1999, the Company obtained approximately $525,000
under a loan agreement (the Loan) from new and existing investors
(the New Investors). As of December 13, 1999, the Company sold an
aggregate of $5.1 million principal amount of 8% notes (the
"Notes") to purchasers in a private placement transaction.
Including the Notes issued upon conversion of the debt issued to
the Company's Chief Executive Officer and President (Note 22(f))
and the Loan issued to New Investors, there is $7.1 million
principal amount of Notes outstanding. These Notes earn interest
semi-annually at 8% per annum, mature on November 30, 2002 and
are convertible into the Company's common stock at an initial
conversion price of $.60 per share.

In connection with the offering of the Notes, Forum and the
entities advised by Pecks entered into a Subordination and
Intercreditor Agreement with the Company and the representative
of the purchasers of the Notes whereby, among other things, they
agreed to subordinate their loan (Note 22(e)) to the Notes,
subject to certain conditions. Also in connection with this
offering, the Company agreed to issue warrants to purchase an
aggregate of 2.75 million shares of the Company's common stock to
designees of Pecks and Forum. These warrants are exercisable from
December 31, 2000 until December 31, 2002 at $.60 per share.

The Notes permit the noteholders' representative to declare an
event default, among other things, if the Company fails to
maintain, as of the last day of any calendar month, consolidated
cash on hand (and cash equivalents and marketable securities) of
at least $1.5 million. As of January 31, 2000, the Company met
this requirement. If an event of default under the Notes were
declared and not cured in the requisite time period, then the
respective representatives of the Notes, Forum and the entities
advised by Pecks could declare their debt securities immediately
due and payable, in which case the Company may be required to
sell substantial assets to raise funds for this repayment and, if
the proceeds of those sales together with any other funds
available are insufficient, the Company could be forced to
declare bankruptcy.

(b)   Net Loss per Common Share

The Company applies SFAS No. 128, Earnings per Share, in
calculating earnings per share. Basic net loss per share is
computed by dividing net loss applicable to common stockholders
by the weighted average number of common shares outstanding
during the period. Diluted net loss per share for the periods
presented is the same as basic net loss per share as the
inclusion of the potential common stock equivalents would be
antidilutive. Antidilutive securities which consist of stock
options, warrants and convertible preferred stock (on an
as-converted basis) that are not included in diluted net loss per
common share were 12,568,143 and 29,510,050 for the nine-month
periods ended September 30, 1998 and 1999, respectively.

(c)   Comprehensive Loss

The Company follows the provisions of SFAS No. 130, Reporting
Comprehensive Income. Comprehensive loss is defined as the change
in equity of a business enterprise during a period from
transactions and other events and circumstances from nonowner
sources. The Company's comprehensive loss is the same as the
reported net loss for all periods presented.

F-28

(d)   Cash Equivalents

The Company considers all highly liquid investments with
maturities of three months or less to be cash equivalents. Cash
and cash equivalents at September 30, 1999 consisted of the
following (at amortized cost, which approximates fair market
value):

September 30, 1999
Cash and cash equivalents-
Cash and money market funds                            $407,966
Corporate bond                                           92,213
$500,179

(e)   Note Payable to Lenders

During November 1998, the Company entered into a $6,000,000 note
payable with Forum Capital Markets, LLC (Forum) and certain
investors associated with Pecks Management Partners Ltd.
(collectively, the Lenders). The terms of the note payable are as
follows: (i) the maturity is November 30, 2003; (ii) the interest
rate is 6%; (iii) interest is payable monthly in arrears, with
the principal due in full at maturity of the loan; (iv) the note
payable is convertible, at the Lender's option, in whole or in
part, into shares of common stock at a rate equal to $2.40 per
share; (v) the note includes a minimum liquidity covenant of
$2,000,000; and (vi) the note payable may not be prepaid, in
whole or in part, at any time prior to December 1, 2000. The
Company has received waivers of noncompliance with the minimum
tangible net worth covenant and for the minimum liquidity
covenant through November 30, 1999. The Company has classified
the outstanding balance of $6,000,000 at September 30, 1999 and
December 31, 1998 as a current liability in the accompanying
consolidated balance sheet as it does not expect to remain in
compliance with the financial covenants. In connection with
refinancing the note payable to a bank, Forum received $400,000,
which was reinvested by Forum to purchase 160,000 shares of
common stock with 40,000 attached warrants at an exercise price
of $3.00 per share. The Company has recorded the $400,000 as a
deferred financing cost, which will be amortized to interest
expense over the term of the note. In addition, Forum received
warrants to purchase 133,333 shares of common stock of the
Company at $3.00 per share. The Company computed the value of the
warrants to be $85,433, by using the Black-Scholes option pricing
model. The Company has recorded this $85,433 as a deferred
financing cost, which will be amortized to interest expense over
the term of the note.

(f)   Related Party Promissory Notes Payable

During September 1999, the Company entered into two $500,000
promissory notes payable with the Company's Chief Executive
Officer and President (the Lender). The terms of the promissory
notes payable are as follows: (i) the maturity is March 1, 2000,
subject to certain conditions, as defined; (ii) interest is
payable at the option of the Lender at either (a) 12% payable in
cash; or (b) 15% payable in common stock of the Company at $.50
per share; (iii) interest is payable monthly in arrears,
beginning October 1, 1999; and (iv) the term note may be prepaid
in whole or in part, at any time without penalty. The promissory
notes payable are secured by substantially all tangible and
intangible assets of the Company.

In November 1999, the Company entered into an additional $500,000
promissory note payable with the Lender. This promissory note
payable had the same terms as the two promissory notes payable
entered into in September 1999. In December 1999, all of the

F-29

promissory notes payable, together with $40,000 in accrued
interest, were converted into 8% notes of the Company due 2002
(Note 22(a)).

(g)   Stock Option Repricing

In September 1999, the Company's Board of Directors authorized a
repricing of all outstanding stock options. Under the terms of
the repricing, all current option holders (5,251,827 shares) had
their options repriced to an exercise price of $.50 per share.
Under Accounting Principles Board Opinion No. 25, the Company is
required to use variable plan accounting for these options until
their expiration or exercise.

(h)   Accrued Expenses

Accrued expenses as of September 30, 1999 consist of the
following:

Interest                                       $     58,770
Payroll and related costs                           987,235
Outside research and clinical costs                 181,375
Professional fees                                   212,812
Research and development costs                      200,000
Sales taxes                                         125,000
Other                                               624,612
--------
$2,389,804

(i)   Supplemental Disclosure of Cash Flow Information

Supplemental disclosure of cash flow information for the nine
month periods ended September 30, 1998 and 1999 are as follows:

Nine Months Ended
September 30,
1998           1999

Supplemental Disclosure of Cash Flow Information:

Cash paid during the period for interest                         $    1,494,323   $       --

Supplemental Disclosure of Noncash Activities:
Accretion of Series A convertible preferred stock dividends      $    1,026,500   $3,193,851
$           --   $1,000,000
Issuance of common stock in lieu of services

Issuance of Series A convertible preferred stock and attached
warrants in exchange for conversion of 9% convertible
subordinated notes payable and accrued interest             $   51,055,850   $       --

Issuance of common stock and attached warrants in exchange for
conversion of convertible promissory notes payable          $    4,800,000   $       --

Issuance of common stock and attached warrants in exchange for
conversion of accounts payable and other obligations        $    5,934,558   $       --

Conversion of Series A convertible preferred stock into shares
of common stock                                             $           --   $      496

F-30

No person has been authorized to give any information or to make any
representations other than those contained in this Prospectus, and, if given or
made, such information or representations must not be relied upon as having been
authorized. This Prospectus does not constitute an offer to sell or the
solicitation of an offer to buy any securities other than the securities to
which it relates or an offer to sell or the solicitation of an offer to buy such
securities in any circumstances in which such offer or solicitation is unlawful.
Neither the delivery of this Prospectus nor any sale made hereunder shall, under
any circumstances, create any implication that there has been no change in the
affairs of Hybridon since the date hereof or that the information contained
herein is correct as of any time subsequent to its date. ---------------
HYBRIDON, INC.

662,167 SHARES
SERIES A CONVERTIBLE PREFERRED STOCK
($.01 par value per share)

34,727,717 SHARES
COMMON STOCK
($.001 par value per share)

PART II
INFORMATION NOT REQUIRED IN PROSPECTUS

Item 13.  Other Expenses of Issuance and Distribution.

Estimated expenses (other than underwriting discounts and commissions)
payable in connection with the sale of the shares of Series A convertible
preferred stock, $.01 par value per share (the "Convertible Preferred Stock")
and shares of common stock, $.001 par value per share (the "Common Stock" and,
together with the Convertible Preferred Stock, the "Securities") offered hereby
are as follows:

SEC Registration fee................................
Printing and engraving expenses.....................
Legal fees and expenses.............................
Accounting fees and expenses........................
Blue Sky fees and expenses
(including legal fees)............................
Transfer agent and registrar fees
and expenses......................................
Miscellaneous.......................................
Total.....................................

The Registrant will bear all expenses shown above.

Item 14.  Indemnification of Directors and Officers.

Article EIGHTH of the Registrant's Restated Certificate of
Incorporation provides that no director of the Registrant shall be personally
liable for any monetary damages for any breach of fiduciary duty as a director,
except to the extent that the Delaware General Corporation Law prohibits the
elimination or limitation of liability of directors for breach of fiduciary
duty.

Article NINTH of the Registrant's Restated Certificate of Incorporation
provides that a director or officer of the Registrant (a) shall be indemnified
by the Registrant against all expenses (including attorneys' fees), judgments,
fines and amounts paid in settlement incurred in connection with any litigation
or other legal proceeding (other than an action by or in the right of the
Registrant) brought against him by virtue of his position as a director or
officer of the Registrant if he acted in good faith and in a manner he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, and, with respect to any criminal action or proceeding, had no
reasonable cause to believe his conduct was unlawful and (b) shall be
indemnified by the Registrant against all expenses (including attorneys' fees)
and amounts paid in settlement incurred in connection with any action by or in
the right of the Registrant brought against him by virtue of his position as a
director or officer of the Registrant if he acted in good faith and in a manner
he reasonably believed to be in, or not opposed to, the best interests of the
Registrant, except that no indemnification shall be made with respect to any
matter as to which such person shall have been adjudged to be liable to the
Registrant, unless a court determines that, despite such adjudication but in
view of all of the circumstances, he is entitled to indemnification of such
expenses. Notwithstanding the foregoing, to the extent that a director or
officer has been successful, on the merits or otherwise, including, without
limitation, the dismissal of an action without prejudice, he is required to be
indemnified by the Registrant against all expenses (including attorneys' fees)
incurred in connection therewith. Expenses shall be advanced to a director or
officer at his request, provided that he undertakes to repay the amount advanced
if it is ultimately determined that he is not entitled to indemnification for
such expenses.

II-1

Indemnification is required to be made unless the Registrant determines
that the applicable standard of conduct required for indemnification has not
been met. In the event of a determination by the Registrant that the director or
officer did not meet the applicable standard of conduct required for
indemnification, or if the Registrant fails to make an indemnification payment
within 60 days after such payment is claimed by such person, such person is
permitted to petition the court to make an independent determination as to
whether such person is entitled to indemnification. As a condition precedent to
the right of indemnification, the director or officer must give the Registrant
notice of the action for which indemnity is sought and the Registrant has the
right to participate in such action or assume the defense thereof.

Article NINTH of the Registrant's Restated Certificate of Incorporation
further provides that the indemnification provided therein is not exclusive, and
provides that in the event that the Delaware General Corporation Law is amended
to expand the indemnification permitted to directors or officers the Registrant
must indemnify those persons to the full extent permitted by such law as so
amended.

Section 145 of the Delaware General Corporation Law provides that a
corporation has the power to indemnify a director, officer, employee or agent of
the corporation and certain other persons serving at the request of the
corporation in related capacities against amounts paid and expenses incurred in
connection with an action or proceeding to which he is or is threatened to be
made a party by reason of such position, if such person shall have acted in good
faith and in a manner he reasonably believed to be in or not opposed to the best
interests of the corporation, and, in any criminal proceeding, if such person
had no reasonable cause to believe his conduct was unlawful; provided that, in
the case of actions brought by or in the right of the corporation, no
indemnification shall be made with respect to any matter as to which such person
shall have been adjudged to be liable to the corporation unless and only to the
extent that the adjudicating court determines that such indemnification is
proper under the circumstances.

Hybridon is a party to an indemnification agreement with Mr. Grinstead.
Such agreement provides that Mr. Grinstead shall be indemnified by the
Registrant (a) against all expenses (as defined in the agreement), judgments,
fines, penalties and amounts paid in settlement actually and reasonably incurred
in connection with any legal proceeding (other than one brought by or on behalf
of the Registrant) if Mr. Grinstead acted in good faith and in a manner which he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, and, with respect to any criminal proceeding, had no reasonable
cause to believe that his conduct was unlawful and (b) against all expenses and
amounts paid in settlement actually and reasonably incurred in connection with a
legal proceeding brought by or on behalf of the Registrant if he acted in good
faith and in a manner which he reasonably believed to be in, or not opposed to,
the best interests of the Registrant, except that no indemnification shall be
made in respect of any claim, issue or matter as to which Mr. Grinstead has been
adjudged to be liable. If, with respect to such proceedings, Mr. Grinstead is
successful on the merits or otherwise, he shall be reimbursed for all expenses.
Mr. Grinstead is required to provide notice to the Registrant of any threatened
or pending litigation, and the Registrant has the right to participate in such
action or assume the defense thereof.

Hybridon has obtained directors and officers insurance for the benefit
of its directors and its officers.

Item 15.  Recent Sales of Unregistered Securities.

In the three years preceding the filing of this registration statement,
Hybridon has issued and sold its Common Stock, warrants to purchase its Common
Stock, Convertible Subordinated Notes and Series A Convertible Preferred Stock,
to certain investors in transactions that were not registered under the
Securities Act of 1933, as amended (the "Securities Act"):

II-2

Unregistered Offerings Pursuant to Section 4(2) Under the 1933 Act

The securities issued in each of the following transactions (items (1)
through (10)) were offered and sold in reliance upon the exemption from
registration under Section 4(2) of the Securities Act, relating to sales by an
issuer not involving a public offering. The securities issued in each of the
following transactions were offered and sold solely to persons who were
"accredited investors" as that term is defined in Regulation D promulgated under
the Securities Act.

(1) On January 20, 1997, Hybridon issued 25,000 shares of Common Stock
to an investment bank as compensation under a financial advisory services
agreement dated that date. These shares were offered and sold to an "accredited
investor" (as that term is defined in Regulation D promulgated under the
Securities Act) in reliance upon the exemption from registration under Section
4(2) of the Securities Act relating to sales by an issuer not involving any
public offering.

(2) On January 25, 1997, Hybridon sold 1,650 shares of Common Stock to
one investor upon exercise by such investor of warrants to purchase Common Stock
for an aggregate purchase price of $9,075. These shares were offered and sold to
an "accredited investor" (as that term is defined in Regulation D promulgated
under the Securities Act) in reliance upon the exemption from registration under
Section 4(2) of the Securities Act relating to sales by an issuer not involving
any public offering.

(3) On April 2, 1997, Hybridon issued to an investment bank $50,000,000
of its 9% Notes. These 9% Notes were offered and sold to an "accredited
investor" (as that term is defined in Regulation D promulgated under the
Securities Act) in reliance upon the exemption from registration under Section
4(2) of the Securities Act relating to sales by an issuer not involving any
public offering.

(4) On April 2, 1997, Hybridon issued to an investment bank warrants to
purchase 71,301 shares of Common Stock at an exercise price of $35.0625 per
share. These warrants were offered and sold to an "accredited investor" (as that
term is defined in Regulation D promulgated under the Securities Act) in
reliance upon the exemption from registration under Section 4(2) of the
Securities Act relating to sales by an issuer not involving any public offering.

(5) On December 10, 1997, Hybridon issued to Dr. Paul Zamecnik, a
Director of Hybridon, 50,000 shares of Common Stock of Hybridon.

(6) On May 5, 1998, Hybridon accepted $48,694,000 principal amount of
its 9% Notes tendered to Hybridon in exchange for 510,505 shares of Series A
preferred stock (the "Series A Preferred Stock") and warrants (the "Class A
Warrants") to purchase 3,002,958 shares of common stock, par value $.001 per
share (the "Common Stock"), of Hybridon (the "Exchange Offer"). As a result of
the Exchange Offer, there is approximately $1.3 million principal amount of the
9% Notes outstanding.

Pursuant to the Exchange Offer, which commenced on February 6, 1998,
all tendering Noteholders received per $1,000 principal amount of the 9% Notes
(including accrued but unpaid interest on the 9% Notes) (i) 10 shares of Series
A Preferred Stock and (ii) Class A Warrants to purchase such number of shares of
Common Stock equal to 25% of the number of shares of Hybridon's Common Stock
into which the Series A Preferred Stock issued to such Noteholder pursuant to
the Exchange Offer would be convertible.

The Convertible Preferred Stock ranks, as to dividends and liquidation
preference, senior to Hybridon's Common Stock. The Convertible Preferred Stock
issued in the Exchange Offer and in the Regulation D Offering, as defined below,
as well as the Convertible Preferred Stock that was issued as a dividend on
September 30, 1998, will be convertible into an aggregate of 15,088,200 shares
of Common Stock, subject to adjustment, beginning May 5, 1999.

II-3

The Class A Warrants will be exercisable commencing on May 5, 1999 for
a period of four years thereafter at $4.25 per share of Common Stock, subject to
adjustment. The Class A Warrants are not subject to redemption at the option of
Hybridon under any circumstances.

The Exchange Offer was undertaken by Hybridon as part of Hybridon's new
business plan contemplating a restructuring of its capital structure to reduce
debt service obligations, a significant reduction in its burn rate and an
infusion of additional equity capital.

(7) On May 5, 1998, Hybridon closed a private placement (the
"Regulation D Offering") of (i) 114,285 shares of Series A Preferred Stock,
which sold at $70 per share, and (ii) Class D warrants (the "Class D Warrants")
to purchase 672,273 shares of Hybridon's Common Stock, subject to adjustment,
for an aggregate amount of approximately $8 million.

The Class D Warrants will be exercisable commencing on May 5, 1999
until May 4, 2003 at $2.40 per share of Common Stock, subject to adjustment.

The net proceeds to Hybridon from the Regulation D Offering are
presently used for general corporate purposes, primarily research and product
development activities, including costs of preparing investigational new drug
applications and conducting preclinical studies and clinical trials, the payment
of payroll and other accounts payable and for debt service required under
Hybridon's debt obligations. The amounts actually expended by Hybridon and the
purposes of such expenditures may vary significantly depending upon numerous
factors, including the progress of Hybridon's research, drug discovery and
development programs, the results of preclinical studies and clinical trials,
the timing of regulatory approvals, sales of DNA products and reagents to third
parties manufactured on a custom contract basis by the HSP Division and margins
on such sales, technological advances, determinations as to the commercial
potential of Hybridon's compounds and the status of competitive products. In
addition, expenditures will also depend upon the establishment of collaborative
research arrangements with other companies, the availability of other financing
and other factors. Under certain circumstances, Hybridon may be required to use
net proceeds to repay indebtedness under its bank credit facility.

(8) On May 5, 1998, Hybridon closed a private placement of units (the
"Unit Offering") consisting of (i) 2,754,654 shares of Common Stock, and (ii)
Class C warrants (the "Class C Warrants") to purchase 788,649 shares of Common
Stock, subject to adjustment, which securities were issued in consideration of
the cancellation (or reduction) of accounts payable, capital lease and other
obligations aggregating $5,509,308.

The Class C Warrants are exercisable at $2.40 per share, subject to
adjustment from time to time, until May 4, 2003.

The Common Stock issued pursuant to the Unit Offering and the Common
Stock underlying the Class C Warrants are subject to a "lock-up" period ending
on May 5, 1999, except to the extent such securities are sold or transferred
pursuant to a Registration Statement. After Hybridon files a Registration
Statement under the Securities Act, 75% of each holder's Units and the
underlying securities will be subject to an additional "lock-up" for the first
three months following the effective date of the Registration Statement (the
"Effective Date"); thereafter, 50% of such securities will be subject to an
additional "lock-up" until six months following the Effective Date; and the
remaining 25% of such securities will be "locked-up" until nine months following
the Effective Date.

(9) On May 5, 1998, Hybridon sold to Dr. Paul Zamecnik 100,000 shares
of Common Stock and Class C Warrants to purchase 25,000 shares of Common Stock,
subject to adjustment, for a purchase price of $200,000.

The net proceeds of this offering were used to reduce accounts payable,
capital lease and other obligations.

II-4

(10) On May 5, 1998, Hybridon issued to certain suppliers a total of
362,500 shares of Common Stock and Class C Warrants to purchase a total of
90,625 shares of Common Stock. These issuances were in consideration of (i)
payment to Hybridon of a total of $362.50, the par value of all such issued
Common Stock, and (ii) the subsequent furnishing of specified services to
Hybridon by each supplier. The extent to which the suppliers have completed
performing the specified services varies.

(11) On December 12, 1998, Hybridon issued to Dr. Paul Zamecnik 50,000
shares of Common Stock in recognition of Dr. Zamecnik's extraordinary
contribution to Hybridon.

(12) On April 16, 1999, Hybridon issued to Pillar Investments Limited
300,000 shares of Common Stock in connection with Pillar's efforts in assisting
Hybridon with restructuring its balance sheet.

(13) On May 1, 1999, Hybridon issued to Forum Capital Markets LLC
160,000 shares of Common Stock and warrants to purchase 173,333 shares of Common
Stock, as a reinvestment by Forum of a $400,000 fee paid to Forum in connection
with the purchase of a bank loan to Hybridon.

The Common Stock issued to Dr. Paul Zamecnik and to the certain
suppliers and the Common Stock underlying the Class C Warrants issued to such
persons are subject to a "lock-up" period ending on May 5, 1999, except to the
extent such securities are sold or transferred pursuant to a Registration
Statement. After Hybridon files a Registration Statement under the Securities
Act, 75% of each holder's Units and the underlying securities will be subject to
an additional "lock-up" for the first three months following the Effective Date;
thereafter, 50% of such securities will be subject to an additional "lock-up"
until six months following the Effective Date; and the remaining 25% of such
securities will be "locked-up" until nine months following the Effective Date.

(14) In October 1999, Hybridon sold approximately $455,000 principal
amount of promissory notes at face value to certain "accredited investors," in
reliance upon the exemption from registration under Section 4(2) of the
Securities Act relating to sales by an issuer not involving any public offering.

(15) In September and November 1999, Hybridon sold an aggregate of $1.5
million principal amount of promissory notes at face value to E. Andrews
Grinstead, III, Hybridon's Chief Executive Officer, in reliance upon the
exemption from registration under Section 4(2) of the Securities Act relating to
sales by an issuer not involving any public offering.

(16) On December 13, 1999, Hybridon sold an aggregate of $5.1 million
principal amount of 8% Notes to purchasers in a private placement transaction.
These 8% Notes were offered and sold to "accredited investors" in reliance upon
the exemption from registration under Section 4(2) of the Securities Act
relating to sales by an issuer not involving any public offering.

(17) As of December 31, 1999, the $455,000 indebtedness under the
October 1999 loan agreement were converted into 8% Notes, in reliance upon the
exemption from registration under Section 4(2) of the Securities Act relating to
sales by an issuer not involving any public offering.

(18) As of December 31, 1999, the $1.5 million principal amount of
promissory notes held by Mr. Grinstead, automatically converted into 8% Notes,
in reliance upon the exemption from registration under Section 4(2) of the
Securities Act relating to sales by an issuer not involving any public offering.

(19) As of December 7, 1999, in connection with the Subordination and
Intercreditor Agreement by and among Hybridon, the representative of the
purchasers of the 8% Notes, Forum and the entities advised by Pecks, whereby,
among other things, the $6,000,000 Forum loan was subordinated to the 8% Notes,
Hybridon issued warrants to purchase an aggregate of 2.75 million shares of
Hybridon common stock to designees of Pecks and Forum. These warrants were
offered

II-5

and sold to "accredited investors" in reliance upon the exemption from
registration under Section 4(2) of the Securities Act relating to sales by an
issuer not involving any public offering.

(20) In connection with the December 13, 1999 private placement of 8%
Notes, Hybridon agreed, subject to certain conditions, to issue to Pillar
Investment Limited or its designees, 8% Notes in an aggregate principal amount
equal to 9% of the aggregate principal amount of 8% Notes sold to investors
introduced to Hybridon by Pillar and warrants to purchase an aggregate principal
amount of 8% Notes equal to 10% of the 8% Notes sold to investors introduced to
Hybridon by Pillar. These notes and warrants were offered and sold to
"accredited investors" in reliance upon the exemption from registration under
Section 4(2) of the Securities Act relating to sales by an issuer not involving
any public offering.

Unregistered Offerings Pursuant to Regulation S Under the Securities Act

The securities issued by Hybridon in each of the following transactions
were offered and sold in reliance upon an exemption from registration under
Regulation S promulgated under the Securities Act, relating to sales by an
issuer in offshore transactions (the "Regulation S Offerings"). The securities
issued in each of the following Regulation S Offerings were offered and sold
solely to persons who were "accredited investors" as that term is defined in
Regulation D promulgated under the Securities Act.

(21) On January 15, 1998, Hybridon commenced a private placement of
units (the "Units"), each Unit consisting of 14% Convertible Subordinated Notes
Due 2007 (the "14% Notes") and warrants (the "Equity Warrants") to purchase
shares of Hybridon's Common Stock (the "14% Note Offering"). The 14% Notes were
subject to both mandatory and optional conversion into shares of Series B
preferred stock, under certain circumstances which, in turn, were convertible
into Common Stock (the "Series B Preferred Stock").

On January 23, 1998, as part of the 14% Note Offering, Hybridon sold
$2,230,000 in principal amount of 14% Notes and Equity Warrants.

On February 9, 1998, as part of the 14% Note Offering, Hybridon sold
$2,384,000 in principal amount of 14% Notes and Equity Warrants.

On March 27, 1998, as part of the 14% Note Offering, Hybridon sold
$200,000 in principal amount of 14% Notes and Equity Warrants.

On April 21, 1998, as part of the 14% Note Offering, Hybridon sold
$300,000 in principal amount of 14% Notes and Equity Warrants.

On April 24, 1998, as part of the 14% Note Offering, Hybridon sold
$1,020,000 in principal amount of 14% Notes and Equity Warrants.

In each of the above closings, the 14% Notes were issued at face value.

(22) On May 5, 1998, Hybridon closed a private placement of 3,223,000
shares of Common Stock and Class B warrants to purchase 805,750 shares of
Hybridon's Common Stock, subject to adjustment, for aggregate gross proceeds of
$6,446,000.

The Class B warrants are exercisable for a period of five years at
$2.40 per share of Common Stock, subject to adjustment from time to time.

The Common Stock underlying the Class B Warrants issued in such private
placement are subject to a "lock-up" for a period ending on May 5, 1999, except
to the extent such securities are sold or transferred pursuant to a Registration
Statement filed by Hybridon under the Securities Act. After Hybridon files a
Registration Statement under the Securities, 75% of each holder's Common Stock

II-6

underlying the Class B and Class C warrants will be subject to an additional
"lock-up" for the first three months following the Effective Date; thereafter,
50% of such securities will be subject to an additional "lock-up" until six
months following the Effective Date; and the remaining 25% of such securities
will be "locked-up" until nine months following the Effective Date.

(23) Hybridon has exchanged all of the 14% Notes issued, including any
right to interest thereon, and all Equity Warrants issued together with the 14%
Notes, for 3,157,322 shares of Common Stock and Class B Warrants to purchase
947,195 shares of Common Stock.

The net proceeds to Hybridon from these offerings were used and
continue to be used for general corporate purposes, primarily research and
product development activities, including costs of preparing investigational new
drug applications and conducting preclinical studies and clinical trials, the
payment of payroll and other accounts payable and for debt service required
under Hybridon's debt obligations. The amounts actually expended by Hybridon and
the purposes of such expenditures may vary significantly depending upon numerous
factors, including the progress of Hybridon's research, drug discovery and
development programs, the results of preclinical studies and clinical trials,
the timing of regulatory approvals, sales of DNA products and reagents to third
parties manufactured on a custom contract basis by the HSP Division and margins
on such sales, technological advances, determinations as to the commercial
potential of Hybridon's compounds and the status of competitive products. In
addition, expenditures will also depend upon the establishment of collaborative
research arrangements with other companies, the availability of other financing
and other factors. Under certain circumstances, Hybridon may be required to use
net proceeds to repay indebtedness under its bank credit facility.

Item 16.  Exhibits and Financial Statement Schedules

(a)     Exhibits:

EXHIBIT INDEX

Exhibit No. Description

3.1(1)   Restated Certificate of Incorporation of the Registrant, as amended.

3.2(2)   Amended and Restated Bylaws of the Registrant.

3.3(3)   Form of Certificate of Designation of Series A Preferred Stock.

3.4(3)   Form of Certificate of Designation of Series B Preferred Stock.

4.1(2)   Specimen Certificate for shares of Common Stock, $.001 par value, of
the Registrant.

4.2(4)   Indenture dated as of March 26, 1997 between Forum Capital Markets LLC
and the Registrant.

4.3(7)   Certificate of Designation of Series A Preferred Stock, par value $.01
per share, dated May 5, 1998.

4.4(7)   Class A Warrant Agreement dated May 5, 1998.

4.5(7)   Class B Warrant Agreement dated May 5, 1998.

4.6(7)   Class C Warrant Agreement dated May 5, 1998.

4.7(7)   Class D Warrant Agreement dated May 5, 1998.

II-7

4.8         Form of Class F Warrant to purchase shares of Common Stock of
Hybridon.

4.9         Form of Notes due 2002 of Hybridon.

4.10        Form of Class G Warrant for the purchase of Notes due 2002 of
Hybridon.

+10.1(2)    License Agreement dated February 21, 1990 and restated as of
September 8, 1993 between the Registrant and the Worcester
Foundation for Biomedical Research, Inc., as amended.

+10.2(2)    Patent License Agreement dated September 21, 1995 between the
Registrant and National Institutes of Health.

+10.3(2)    Patent License Agreement effective as of October 13, 1994 between
the Registrant and McGill University.

+10.4(2)    License Agreement effective as of October 25, 1995 between the
Registrant and the General Hospital Corporation.

+10.5(2)    License Agreement dated as of October 30, 1995 between the
Registrant and Yoon S. Cho-Chung.

+10.6(2)    Collaborative Study Agreement effective as of December 30, 1992
between the Registrant and Medtronic, Inc.

+10.7(2)    System Design and Procurement Agreement dated as of December 16,
1994 between the Registrant and Pharmacia Biotech, Inc.

10.8(2)     Lease dated March 10, 1994 between the Registrant and Laborer's
Pension/Milford Investment Corporation for space located at 155
Fortune Boulevard, Milford, Massachusetts, including Note in the
original principal amount of $750,000.

10.9(2)     Registration Rights Agreement dated as of February 21, 1990 between
the Registrant, the Worcester Foundation for Biomedical Research,
Inc. and Paul C. Zamecnik.

10.10(2)    Registration Rights Agreement dated as of June 25, 1990 between the
Registrant and Nigel L. Webb.

10.11(2)    Registration Rights Agreement dated as of February 6, 1992 between
the Registrant and E. Andrews Grinstead, III.

10.12(2)    Registration Rights Agreement dated as of February 6, 1992 between
the Registrant and Anthony J. Payne.

++10.13(2)  1990 Stock Option Plan, as amended.

++10.14(2)  1995 Stock Option Plan.

++10.15(2)  1995 Director Stock Plan.

++10.16(2)  1995 Employee Stock Purchase Plan.

10.17(2)    Form of Warrant originally issued to Pillar Investment Limited to
purchase shares of Common Stock issued as placement commissions in
connection with the sale of shares of

II-8

Series F Convertible Preferred Stock and in consideration of
financial advisory service, as amended.

10.18(2)    Warrant issued to Pillar S.A. to purchase 100,000 shares of Common
Stock dated as of March 1, 1994, as amended.

10.19(2)    Warrant issued to Pillar S.A. to purchase 100,000 shares of Common
Stock dated as of March 1, 1995.

10.20(2)    Form of Warrant issued to Pillar Investment Limited to purchase
shares of Common Stock issued as placement commissions in connection
with the sale of Units pursuant to the Series G Agreement.

++10.21(5)  Employment Agreement dated as of March 1, 1997 between the
Registrant and E. Andrews Grinstead, III.

10.22(2)    Indemnification Agreement dated as of February 6, 1992 between the
Registrant and E. Andrews Grinstead, III.

++10.23(6)  Employment Agreement dated March 1, 1997 between the Registrant and
Dr. Sudhir Agrawal.

++10.24(2)  Consulting Agreement dated as of February 21, 1990 between the
Registrant and Dr. Paul C. Zamecnik.

10.25(2)    Master Lease Agreement dated as of March 1, 1994 between the
Registrant and General Electric Capital Corporation.

+10.26(6)   Research, Development and License Agreement dated as of January 24,
1996 between the Registrant and G.D. Searle & Co.

+10.27(6)   Manufacturing and Supply Agreement dated as of January 24, 1996
between the Registrant and G.D. Searle & Co.

10.28(6)    Registration Rights Agreement dated as of January 24, 1996 between
the Registrant and G.D. Searle & Co.

10.29(5)    Loan and Security Agreement dated as of December 31, 1996 between
the Registrant and Silicon Valley Bank.

10.30(7)    First Amendment to Loan and Security Agreement dated March 30, 1998
between Hybridon, Inc. and Silicon Valley Bank.

10.31(8)    Second Amendment to Loan and Security Agreement dated May 19, 1998,
effective as of April 30, 1998, between Hybridon, Inc. and Silicon
Valley Bank.

10.32(9)    Third Amendment to Loan and Security Agreement dated September 18,
1998 between Hybridon, Inc. and Silicon Valley Bank.

10.33(9)    Fourth Amendment to Loan and Security Agreement dated October 30,
1998, effective as of September 29, 1998 between Hybridon, Inc. and
Silicon Valley Bank.

10.34(12)   Fifth Amendment to Loan and Security Agreement dated December 4,
1998 between Hybridon, Inc. and Silicon Valley Bank.

II-9

10.35(5)    Warrant issued to Silicon Valley Bank to purchase 65,000 shares of
Common Stock dated as of December 31, 1996.

10.36(5)    Registration Rights Agreement dated as of December 31, 1996 between
the Registrant and Silicon Valley Bank.

+10.37(5)   Supply and Sales Agreement dated as of September 1, 1996 between the
Registrant and P.E. Applied Biosystems.

10.38(2)    Registration Rights Agreement dated as of March 26, 1997 between
Forum Capital Markets LLC and the Registrant.

10.39(2)    Warrant Agreement dated as of March 26, 1997 between Forum Capital
Markets LLC and the Registrant.

+10.40(6)   Amendment No. 1 to License Agreement, dated as of February 21, 1990
and restated as of September 8, 1993, by and between the Worcester
Foundation for Biomedical Research, Inc. and the Registrant, dated
as of November 26, 1996.

10.41(10)   Letter Agreement dated May 12, 1997 between the Registrant and
Pillar S.A. amending the Consulting Agreement dated as of March 1,
1994 between the Registrant and Pillar S.A.

10.42(10)   Amendment dated July 15, 1997 to the Series G Convertible Preferred
Stock and Warrant Purchase Agreement dated as of September 9, 1994
among the Registrant and certain purchasers, as amended.

10.43(1)    Consent Agreement dated January 15, 1998 between Silicon Valley Bank
and the Registrant relating to the Silicon Agreement.

10.44(11)   Letter Agreement between the Registrant and Forum Capital Markets
LLC and Pecks Management Partners Ltd. for the purchase of the Loan
and Security Agreement with Silicon Valley Bank.

10.45(7)    Financial Advisory Agreement between Registrant and Pillar
Investments Ltd. dated May 5, 1998.

10.46(7)    Placement Agency Agreement between Registrant and Pillar Investments
Ltd. dated as of January 15, 1998.

+++10.47(12) Licensing Agreement dated March 12, 1999 by and between Hybridon,
Inc. and Integrated DNA Technologies, Inc.

+++10.48(13) Licensing Agreement dated September 7, 1999 by and between
Hybridon, Inc. and Genzyme Corporation.

10.49(13)   Form of loan agreement relating to a loan in the amount of $454,901
made to Hybridon, Inc. in October 1999 by various parties.

10.50(13)   Form of promissory note relating to a loan in the amount of $454,901
made to Hybridon, Inc. in October 1999 by various parties.

10.51(13)   Loan Agreement dated as of September 1, 1999, between Hybridon, Inc.
and E. Andrews Grinstead, III.

II-10

10.52(13)   Term promissory note in the amount of $500,000 dated September 1,
1999, by Hybridon, Inc. in favor of E. Andrews Grinstead, III.

10.53(13)   Term promissory note in the amount of $500,000 dated September 27,
1999, by Hybridon, Inc. in favor of E. Andrews Grinstead, III.

10.54       Subordination and Intercreditor Agreement by and among Hybridon, the
holders of Notes due 2002, Forum and entities advised by Pecks,
dated as of December 7, 1999.

10.55       Letter Agreement between Hybridon and Pillar Investments dated
December 10, 1999.

10.56       Form of Subscription Agreements dated as of December 13, 1999, by
and among Hybridon and the purchasers of Notes due 2002.

21.1(2)     Subsidiaries of the Registrant.

23.1        Consent of Arthur Andersen LLP.

23.2(11)    Consent of McDonnell Boehnen Hulbert & Berghoff.

------------------------------------------------

(1)         Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1997.

(2)         Incorporated by reference to Exhibits to the Registrant's
Registration Statement on Form S-1 (File No. 33-99024).

(3)         Incorporated by reference to Exhibit 9(a)(1) to the Registrant's
Schedule 13E-4 dated February 6, 1998.

(4)         Incorporated by reference to Exhibits to the Registrant's Current
Report on Form 8-K dated April 2, 1997.

(5)         Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1996.

(6)         Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1995.

(7)         Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended March 31, 1998.

(8)         Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended June 30, 1998.

(9)         Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended September 30, 1998.

(10)        Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended June 30, 1997.

II-11

(11)        Incorporated by reference to Exhibits to the Registrant's
Registration Statement on Form S-1 (File No. 333-69649).

(12)        Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1998.

(13)        Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended September 30, 1999.

+           Confidential treatment granted as to certain portions, which
portions are omitted and filed separately with the Commission.

++          Management contract or compensatory plan or arrangement required to
be filed as an Exhibit to the Annual Report on Form 10-K for the
year ended December 31, 1997.

+++         Confidential treatment requested as to certain portions, which
portions are omitted and filed separately with the Commission.

Item 17.  Undertakings.

Insofar as indemnification for liabilities arising under the Securities
Act may be permitted to directors, officers and controlling persons of the
Registrant pursuant to provisions described in Item 14 above, or otherwise, the
Registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer or controlling person in
connection with the securities being registered, the Registrant will, unless in
the opinion of its counsel the matter has been settled by controlling precedent,
submit to a court of appropriate jurisdiction the question whether such
indemnification by it is against public policy as expressed in the Securities
Act and will be governed by the final adjudication of such issue.

The undersigned registrant hereby undertakes:

(1)      To file, during any period in which offers or sales are being made, a
post-effective amendment to this Registration Statement;

(2)      To include any Prospectus required by Section 10(a)(3) of the
Securities Act of 1933;

(3)      To reflect in the Prospectus any facts or events arising after the
effective date of the Registration Statement (or the most recent
post-effective amendment thereof) which, individually or in the
aggregate, represent a fundamental change in the information set forth
in the Registration Statement. Notwithstanding the foregoing, any
increase or decrease in volume of Securities offered (if the total
dollar value of securities offered would not exceed that which was
registered) and any deviation from the low or high end of the estimated
maximum offering range may be reflected in the form of prospectus filed
with the Commission pursuant to Rule 424(b) if, in the aggregate, the
changes in volume and price represent no more than 20 percent change in
the maximum aggregate offering price set forth in the "Calculation of
Registration Fee" table in the effective registration statement;

(4)      To include any material information with respect to the plan of
distribution not previously disclosed in the Registration Statement or
any material change to such information in the Registration Statement;

II-12

(5)      That, for the purpose of determining any liability under the Securities
Act, each such post-effective amendment shall be deemed to be a new
Registration Statement relating to the Securities offered therein, and
the offering of such securities at that time shall be deemed to be the
initial bona fide offering thereof;

(6)      To remove from registration by means of a post-effective amendment any
of the Securities being registered which remain unsold at the
termination of the offering.

II-13

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended,
the Registrant has duly caused this Registration Statement to be signed on its
behalf by the undersigned, thereunto duly authorized, on February 15, 2000.

HYBRIDON,  INC.

By: /s/  SUDHIR AGRAWAL                   
----------------------------------------
Sudhir Agrawal
President and Acting Chief Executive 
Officer

Pursuant to the requirements of the Securities Act of 1933, as amended, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.

Signatures              Title(s)                            Date
----------              --------                            ----

President, Acting Chief 
----------------------------    Executive Officer             February    , 2000
Dr. Sudhir Agrawal              and Director                                    

*                               Chairman and Director         February    , 2000
----------------------------
Dr. James B. Wyngaarden

*                               Chief Executive Officer       February    , 2000
----------------------------    and Director
E. Andrews Grinstead, III

/s/ ROBERT G. ANDERSEN          Chief Financial Officer       February    , 2000
-----------------------------   Vice President of
Mr. Robert G. Andersen          Operations and Planning

*                               Director                      February    , 2000
-----------------------------  
Mr. Nasser Menhall

*                               Director                      February    , 2000
-----------------------------  
Dr. Paul C. Zamecnik

*                               Director                      February    , 2000
-----------------------------  
Mr. Youssef El-Zein

-----------------------------   Director                      February    , 2000
Mr. Arthur W. Berry

II-14

*                               Director                      February    , 2000
-----------------------------
Mr. Harold L. Purkey

Director                      February    , 2000
Mr. Camille Chebeir

*        By: /s/ ROBERT G. ANDERSEN
Robert G. Andersen
Attorney-in-fact

II-15

EXHIBIT INDEX

Exhibit No.       Description
-----------       -----------

3.1(1)      Restated Certificate of Incorporation of the Registrant, as amended.

3.2(2)      Amended and Restated Bylaws of the Registrant.

3.3(3)      Form of Certificate of Designation of Series A Preferred Stock.

3.4(3)      Form of Certificate of Designation of Series B Preferred Stock.

4.1(2)      Specimen Certificate for shares of Common Stock, $.001 par value, of
the Registrant.

4.2(4)      Indenture dated as of March 26, 1997 between Forum Capital Markets
LLC and the Registrant.

4.3(7)      Certificate of Designation of Series A Preferred Stock, par value
$.01 per share, dated May 5, 1998.

4.4(7)      Class A Warrant Agreement dated May 5, 1998.

4.5(7)      Class B Warrant Agreement dated May 5, 1998.

4.6(7)      Class C Warrant Agreement dated May 5, 1998.

4.7(7)      Class D Warrant Agreement dated May 5, 1998.

4.8         Form of Class F Warrant to purchase shares of Common Stock of
Hybridon.

4.9         Form of Notes due 2002 of Hybridon.

4.10        Form of Class G Warrant for the purchase of Notes due 2002 of
Hybridon.

+10.1(2)    License Agreement dated February 21, 1990 and restated as of
September 8, 1993 between the Registrant and the Worcester
Foundation for Biomedical Research, Inc., as amended.

+10.2(2)    Patent License Agreement dated September 21, 1995 between the
Registrant and National Institutes of Health.

+10.3(2)    Patent License Agreement effective as of October 13, 1994 between
the Registrant and McGill University.

+10.4(2)    License Agreement effective as of October 25, 1995 between the
Registrant and the General Hospital Corporation.

+10.5(2)    License Agreement dated as of October 30, 1995 between the
Registrant and Yoon S. Cho-Chung.

+10.6(2)    Collaborative Study Agreement effective as of December 30, 1992
between the Registrant and Medtronic, Inc.

+10.7(2)    System Design and Procurement Agreement dated as of December 16,
1994 between the Registrant and Pharmacia Biotech, Inc.

II-16

10.8(2)     Lease dated March 10, 1994 between the Registrant and Laborer's
Pension/Milford Investment Corporation for space located at 155
Fortune Boulevard, Milford, Massachusetts, including Note in the
original principal amount of $750,000.

10.9(2)     Registration Rights Agreement dated as of February 21, 1990 between
the Registrant, the Worcester Foundation for Biomedical Research,
Inc. and Paul C. Zamecnik.

10.10(2)    Registration Rights Agreement dated as of June 25, 1990 between the
Registrant and Nigel L. Webb.

10.11       (2) Registration Rights Agreement dated as of February 6, 1992
between the Registrant and E. Andrews Grinstead, III.

10.12(2)    Registration Rights Agreement dated as of February 6, 1992 between
the Registrant and Anthony J. Payne.

++10.13(2)  1990 Stock Option Plan, as amended.

++10.14(2)  1995 Stock Option Plan.

++10.15(2)  1995 Director Stock Plan.

++10.16(2)  1995 Employee Stock Purchase Plan.

10.17(2)    Form of Warrant originally issued to Pillar Investment Limited to
purchase shares of Common Stock issued as placement commissions in
connection with the sale of shares of Series F Convertible Preferred
Stock and in consideration of financial advisory service, as
amended.

10.18(2)    Warrant issued to Pillar S.A. to purchase 100,000 shares of Common
Stock dated as of March 1, 1994, as amended.

10.19(2)    Warrant issued to Pillar S.A. to purchase 100,000 shares of Common
Stock dated as of March 1, 1995.

10.20(2)    Form of Warrant issued to Pillar Investment Limited to purchase
shares of Common Stock issued as placement commissions in connection
with the sale of Units pursuant to the Series G Agreement.

++10.21(5)  Employment Agreement dated as of March 1, 1997 between the
Registrant and E. Andrews Grinstead, III.

10.22(2)    Indemnification Agreement dated as of February 6, 1992 between the
Registrant and E. Andrews Grinstead, III.

++10.23(6)  Employment Agreement dated March 1, 1997 between the Registrant and
Dr. Sudhir Agrawal.

++10.24(2)  Consulting Agreement dated as of February 21, 1990 between the
Registrant and Dr. Paul C. Zamecnik.

10.25(2)    Master Lease Agreement dated as of March 1, 1994 between the
Registrant and General Electric Capital Corporation.

+10.26(6)   Research, Development and License Agreement dated as of January 24,
1996 between the Registrant and G.D. Searle & Co.

+10.27(6)   Manufacturing and Supply Agreement dated as of January 24, 1996
between the Registrant and G.D. Searle & Co.

II-17

10.28(6)    Registration Rights Agreement dated as of January 24, 1996 between
the Registrant and G.D. Searle & Co.

10.29(5)    Loan and Security Agreement dated as of December 31, 1996 between
the Registrant and Silicon Valley Bank.

10.30(7)    First Amendment to Loan and Security Agreement dated March 30, 1998
between Hybridon, Inc. and Silicon Valley Bank.

10.31(8)    Second Amendment to Loan and Security Agreement dated May 19, 1998,
effective as of April 30, 1998, between Hybridon, Inc. and Silicon
Valley Bank.

10.32(9)    Third Amendment to Loan and Security Agreement dated September 18,
1998 between Hybridon, Inc. and Silicon Valley Bank.

10.33(9)    Fourth Amendment to Loan and Security Agreement dated October 30,
1998, effective as of September 29, 1998 between Hybridon, Inc. and
Silicon Valley Bank.

10.34(12)   Fifth Amendment to Loan and Security Agreement dated December 4,
1998 between Hybridon, Inc. and Silicon Valley Bank.

10.35(5)    Warrant issued to Silicon Valley Bank to purchase 65,000 shares of
Common Stock dated as of December 31, 1996.

10.36(5)    Registration Rights Agreement dated as of December 31, 1996 between
the Registrant and Silicon Valley Bank.

+10.37(5)   Supply and Sales Agreement dated as of September 1, 1996 between the
Registrant and P.E. Applied Biosystems.

10.38(2)    Registration Rights Agreement dated as of March 26, 1997 between
Forum Capital Markets LLC and the Registrant.

10.39(2)    Warrant Agreement dated as of March 26, 1997 between Forum Capital
Markets LLC and the Registrant.

+10.40(6)   Amendment No. 1 to License Agreement, dated as of February 21, 1990
and restated as of September 8, 1993, by and between the Worcester
Foundation for Biomedical Research, Inc. and the Registrant, dated
as of November 26, 1996.

10.41(10)   Letter Agreement dated May 12, 1997 between the Registrant and
Pillar S.A. amending the Consulting Agreement dated as of March 1,
1994 between the Registrant and Pillar S.A.

10.42(10)   Amendment dated July 15, 1997 to the Series G Convertible Preferred
Stock and Warrant Purchase Agreement dated as of September 9, 1994
among the Registrant and certain purchasers, as amended.

10.43(1)    Consent Agreement dated January 15, 1998 between Silicon Valley Bank
and the Registrant relating to the Silicon Agreement.

10.44(11)   Letter Agreement between the Registrant and Forum Capital Markets
LLC and Pecks Management Partners Ltd. for the purchase of the Loan
and Security Agreement with Silicon Valley Bank.

10.45(7)    Financial Advisory Agreement between Registrant and Pillar
Investments Ltd. dated May 5, 1998.

10.46(7)    Placement Agency Agreement between Registrant and Pillar Investments
Ltd. dated as of January 15, 1998.

II-18

+++10.47(12) Licensing Agreement dated March 12, 1999 by and between Hybridon,
Inc. and Integrated DNA Technologies, Inc.

+++10.48(13) Licensing Agreement dated September 7, 1999 by and between
Hybridon, Inc. and Genzyme Corporation.

10.49(13)   Form of loan agreement relating to a loan in the amount of $454,901
made to Hybridon, Inc. in October 1999 by various parties.

10.50(13)   Form of promissory note relating to a loan in the amount of $454,901
made to Hybridon, Inc. in October 1999 by various parties.

10.51(13)   Loan Agreement dated as of September 1, 1999, between Hybridon, Inc.
and E. Andrews Grinstead,III.

10.52(13)   Term promissory note in the amount of $500,000 dated September 1,
1999, by Hybridon, Inc. in favor of E. Andrews Grinstead, III.

10.53(13)   Term promissory note in the amount of $500,000 dated September 27,
1999, by Hybridon, Inc. in favor of E. Andrews Grinstead, III.

10.54       Subordination and Intercreditor Agreement by and among Hybridon, the
holders of Notes due 2002, Forum and entities advised by Pecks,
dated as of December 7, 1999.

10.55       Letter Agreement between Hybridon and Pillar Investments dated
December 10, 1999.

10.56       Form of Subscription Agreements dated as of December 13, 1999, by
and among Hybridon and the purchasers of Notes due 2002.

21.1(2)     Subsidiaries of the Registrant.

23.1        Consent of Arthur Andersen LLP.

23.2(11)    Consent of McDonnell Boehnen Hulbert & Berghoff.

-----------------------------------------------

(1)         Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1997.

(2)         Incorporated by reference to Exhibits to the Registrant's
Registration Statement on Form S-1 (File No. 33-99024).

(3)         Incorporated by reference to Exhibit 9(a)(1) to the Registrant's
Schedule 13E-4 dated February 6, 1998.

(4)         Incorporated by reference to Exhibits to the Registrant's Current
Report on Form 8-K dated April 2, 1997.

(5)         Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1996.

(6)         Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1995.

(7)         Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended March 31, 1998.

II-19

(8)         Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended June 30, 1998.

(9)         Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended September 30, 1998.

(10)        Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended June 30, 1997.

(11)        Incorporated by reference to Exhibits to the Registrant's
Registration Statement on Form S-1 (File No. 333-69649).

(12)        Incorporated by reference to Exhibits to the Registrant's Annual
Report on Form 10-K for the year ended December 31, 1998. =

(13)        Incorporated by reference to Exhibits to the Registrant's Quarterly
Report on Form 10-Q for the period ended September 30, 1999.

+           Confidential treatment granted as to certain portions, which
portions are omitted and filed separately with the Commission.

++          Management contract or compensatory plan or arrangement required to
be filed as an Exhibit to the Annual Report on Form 10-K for the
year ended December 31, 1997.

+++         Confidential treatment requested as to certain portions, which
portions are omitted and filed separately with the Commission.

II-20

NEITHER THESE SECURITIES NOR THE COMMON STOCK ISSUABLE UPON EXERCISE HEREOF HAVE
BEEN  REGISTERED  UNDER  THE  SECURITIES  ACT OF  1933 OR ANY  APPLICABLE  STATE
SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED
OR OTHERWISE  TRANSFERRED IN THE ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT
WITH RESPECT TO THE  SECURITIES  UNDER SUCH ACT OR AN EXEMPTION  THEREFROM.  ANY
SUCH TRANSFER MAY ALSO BE SUBJECT TO APPLICABLE STATE SECURITIES LAWS.

HYBRIDON, INC.

Class F Warrant for the Purchase of Shares of
Common Stock

No. F-2                                              [2,750,000 in total] Shares

FOR VALUE  RECEIVED,  HYBRIDON,  INC., a Delaware  corporation
(the "Company"), hereby certifies that _______________ or its registered assigns
(the  "Holder")  is  entitled  to  purchase  from the  Company,  subject  to the
provisions of this Warrant (the "Warrant"), at any time on or after December 31,
2000 (the "Initial Exercise Date"),  and prior to 5:00 P.M., New York City time,
on  December  31,  2002 (the  "Termination  Date"),  [2,750,000]  fully paid and
non-assessable  shares of the  Common  Stock,  $.001 par value,  of the  Company
("Common Stock"), at an exercise price of $0.60 per share of Common Stock for an
aggregate  exercise  price of [one million six hundred  fifty  thousand  dollars
($1,650,000)]  (the  aggregate  purchase  price  payable for the Warrant  Shares
hereunder  is  hereinafter  sometimes  referred  to as the  "Aggregate  Exercise
Price").  The number of shares of Common Stock to be received  upon  exercise of
this Warrant and the price to be paid for each share of Common Stock are subject
to possible adjustment from time to time as hereinafter set forth. The shares of
Common Stock or other  securities or property  deliverable upon such exercise as
adjusted from time to time is hereinafter  sometimes referred to as the "Warrant
Shares." The exercise price of a share of Common Stock in effect at any time and
as adjusted from time to time is hereinafter  sometimes  referred to as the "Per
Share Exercise  Price." The Per Share Exercise Price is subject to adjustment as
hereinafter provided; in the event of any such adjustment, the number of Warrant
Shares shall also be adjusted,  by dividing the Aggregate  Exercise Price by the
Per Share  Exercise  Price in effect  immediately  after  such  adjustment.  The
Aggregate Exercise Price is not subject to adjustment.

1.       Exercise of Warrant.

(a) This Warrant may be exercised in whole or in part,  at any
time by its holder  commencing  on the  Initial  Exercise  Date and prior to the
Termination Date:

(i) by  presentation  and  surrender of this Warrant,
together with the duly executed  subscription form attached at
the end hereof,  at the address set forth in  Subsection  8(a)
hereof,  together with payment,  by certified or official bank
check or wire transfer payable to the order of the Company, of
the Aggregate Exercise Price or the proportionate part thereof
if exercised in part; or

(ii) by  presentation  and surrender of this Warrant,
together  with  the  duly  executed   cashless  exercise  form
attached  at the end  hereof (a  "Cashless  Exercise")  at the
address set forth in Subsection  8(a) hereof.  The exchange of
Common  Stock for the  Warrant  shall  take  place on the date
specified in the Cashless Exercise Form or, if later, the date
the Cashless  Exercise Form is surrendered to the Company (the
"Exchange  Date").  Such  presentation  and surrender shall be
deemed  a  waiver  of  the  Holder's  obligation  to  pay  the
Aggregate Exercise Price, or the proportionate part thereof if
this Warrant is exercised in part.  In the event of a Cashless
Exercise,  this Warrant shall represent the right to subscribe
for and to  acquire  the  number of  shares  of  Common  Stock
(rounded to the next highest  integer) equal to (x) the number
of shares  of  Common  Stock  specified  by the  Holder in its
Cashless  Exercise Form (the "Total  Number") (such number not
to exceed the maximum number of shares of Common Stock subject
to this  Warrant,  as may be adjusted  from time to time) less
(y) the number of shares of Common Stock equal to the quotient
obtained by dividing  (A) the product of the Total  Number and
the  existing  Per  Share  Exercise  Price by (B) the  Current
Market Price (as defined in Subsection 3(h)).

(b) If this  Warrant is  exercised  in part only,  the Company
shall, upon presentation of this Warrant upon such exercise, execute and deliver
(along with the  certificate  for the Warrant  Shares  purchased)  a new Warrant
evidencing  the  rights of the  Holder  hereof to  purchase  the  balance of the
Warrant  Shares  purchasable  hereunder  upon the same terms and  conditions  as
herein set forth.  Upon proper  exercise of this Warrant,  the Company  promptly
shall deliver certificates for the Warrant Shares to the Holder duly legended as
authorized by the subscription  form. No fractional shares or scrip representing
fractional  shares shall be issued upon exercise of this Warrant;  provided that
the  Company  shall  pay to the  holder  of the  Warrant  cash  in  lieu of such
fractional shares.

2.  Reservation of Warrant Shares;  Fully Paid Shares;  Taxes.
The Company hereby  represents that it has, and until expiration of this Warrant
agrees that it shall,  reserve for  issuance or delivery  upon  exercise of this
Warrant,  such  number of shares of the Common  Stock as shall be  required  for
issuance  and/or delivery upon exercise of this Warrant in full, and agrees that
all Warrant Shares so issued and/or delivered will be validly issued, fully paid
and non-assessable,  and further agrees to pay all taxes and charges that may be
imposed upon such issuance and/or delivery.

3.       Protection Against Dilution.

(a) In case the Company shall  hereafter (i) pay a dividend or
make a  distribution  on its  Common  Stock in  shares  of  Common  Stock,  (ii)
subdivide its outstanding shares of Common Stock into a greater number of shares
or (iii) combine its outstanding shares

2

of Common Stock into a smaller  number of shares  (each of (i) through  (iii) an
"Action"),  the Per Share  Exercise  Price  shall be  adjusted  to be equal to a
fraction,  the numerator of which shall be the Aggregate  Exercise Price and the
denominator  of which  shall be the  number of  shares of Common  Stock or other
capital stock of the Company that the Holder would have held (solely as a result
of the  exercise of this Warrant and the  operation of such Action)  immediately
following  such Action if this Warrant had been exercised  immediately  prior to
such Action.  An adjustment  made pursuant to this  Subsection 3(a) shall become
effective  immediately  after  the  record  date in the  case of a  dividend  or
distribution and shall become effective  immediately after the effective date in
the case of a subdivision, combination or reclassification.

(b)  In  the   event   of  any   capital   reorganization   or
reclassification, or any consolidation or merger to which the Company is a party
other than a merger or  consolidation  in which the  Company  is the  continuing
corporation,  or in case of any sale or  conveyance  to  another  entity  of the
property of the Company as an entirety or  substantially  as an entirety,  or in
the case of any  statutory  exchange  of  securities  with  another  corporation
(including  any  exchange  effected  in  connection  with a  merger  of a  third
corporation  into the Company),  the Holder of this Warrant shall have the right
thereafter  to receive on the  exercise  of this  Warrant the kind and amount of
securities,  cash or other  property  which the Holder  would have owned or have
been   entitled   to   receive    immediately    after   such    reorganization,
reclassification,  consolidation, merger, statutory exchange, sale or conveyance
had this Warrant been exercised  immediately prior to the effective date of such
reorganization,  reclassification,  consolidation,  merger,  statutory exchange,
sale or conveyance  and in any such case, if necessary,  appropriate  adjustment
shall be made in the  application  of the provisions set forth in this Section 3
with  respect  to the  rights  and  interests  thereafter  of the Holder of this
Warrant  to the end that  the  provisions  set  forth  in this  Section  3 shall
thereafter  correspondingly be made applicable,  as nearly as may reasonably be,
in relation to any shares of stock or other  securities  or property  thereafter
deliverable  on the  exercise  of this  Warrant.  The above  provisions  of this
Subsection   3(b)  shall   similarly   apply  to   successive   reorganizations,
reclassifications,   consolidations,  mergers,  statutory  exchanges,  sales  or
conveyances.  The issuer of any shares of stock or other  securities or property
thereafter  deliverable on the exercise of this Warrant shall be responsible for
all of the agreements and obligations of the Company hereunder. A sale of all or
substantially  all of the assets of the Company for a  consideration  consisting
primarily  of  securities  shall be deemed a  consolidation  or  merger  for the
foregoing purposes.

(c)  Whenever  the  Per  Share  Exercise  Price  payable  upon
exercise of each  Warrant is adjusted  pursuant to this Section 3, the number of
shares of Common Stock underlying a Warrant shall  simultaneously be adjusted to
equal the number  obtained  by  dividing  the  Aggregate  Exercise  Price by the
adjusted Per Share Exercise Price.

(d) No  adjustment  in the Per Share  Exercise  Price shall be
required  unless  such  adjustment  would  require an increase or decrease of at
least $0.05 per share of Common Stock;  provided,  however, that any adjustments
which by reason of this  Subsection  3(d) are not  required  to be made shall be
carried  forward  and taken  into  account  in any  subsequent  adjustment.  All
calculations  under this  Section 3 shall be made to the nearest  cent or to the
nearest  1/100th of a share,  as the case may be.  Anything in this Section 3 to
the  contrary  notwithstanding,  the  Company  shall be  entitled  to make  such
reductions in the Per Share  Exercise  Price,  in addition to those  required by
this Section 3, as it in its discretion shall deem to

3

be  advisable  in order  that any  stock  dividend,  subdivision  of  shares  or
distribution   of  rights  to  purchase  stock  or  securities   convertible  or
exchangeable for stock hereafter made by the Company to its  stockholders  shall
not be taxable.

(e)  Whenever  the Per Share  Exercise  Price is  adjusted  as
provided in this Section 3 and upon any  modification  of the rights of a Holder
of Warrants in accordance with this Section 3, the Chief Financial  Officer,  or
equivalent  officer, of the Company shall promptly prepare a certificate setting
forth the Per Share  Exercise  Price and the number of Warrant Shares after such
adjustment or the effect of such  modification,  a brief  statement of the facts
requiring such adjustment or  modification  and the manner of computing the same
and cause copies of such certificate to be mailed to the Holder.

(f) If the Board of Directors of the Company shall declare any
dividend or other  distribution  with respect to the Common  Stock,  the Company
shall  mail  notice  thereof  to the  Holder no fewer  than 30 days prior to the
record date fixed for determining  stockholders  entitled to participate in such
dividend or other distribution.

(g) If, as a result of an  adjustment  made  pursuant  to this
Section 3, the Holder of any Warrant  thereafter  surrendered for exercise shall
become  entitled to receive  shares of two or more  classes of capital  stock or
shares of Common  Stock and other  capital  stock of the  Company,  the Board of
Directors (whose  determination  shall be conclusive and shall be described in a
written  notice to the Holder of any  Warrant  promptly  after such  adjustment)
shall  determine the allocation of the adjusted Per Share Exercise Price between
or among shares or such  classes of capital  stock or shares of Common Stock and
other capital stock.

(h) For the purpose of any computation  under Section 3 above,
the then Current  Market Price per share (the "Current  Market  Price") shall be
deemed to be the last sale price of the Common Stock on the trading day prior to
such date or, in case no such reported sales take place on such day, the average
of the last  reported  bid and asked  prices of the Common Stock on such day, in
either case on the principal  national  securities  exchange on which the Common
Stock is admitted to trading or listed,  or if not listed or admitted to trading
on any such exchange,  the representative  closing bid price of the Common Stock
as reported by the National  Association of Securities  Dealers,  Inc. Automated
Quotations  System  ("NASDAQ"),  or other similar  organization  if NASDAQ is no
longer reporting such information, or if not so available, the fair market value
of the Common Stock as determined  by the  Company's  Board of Directors in good
faith.

4.  Limited  Transferability.  This  Warrant  may not be sold,
transferred,  assigned or  hypothecated  by the Holder except in compliance with
the provisions of the Act and the applicable  state  securities "blue sky" laws,
and is so  transferable  only upon the books of the Company which it shall cause
to be maintained for such purpose.  The Company may treat the registered  Holder
of this  Warrant as he or it appears on the  Company's  books at any time as the
Holder for all purposes. The Company shall permit any Holder of a Warrant or his
duly authorized  attorney,  upon written request during ordinary business hours,
to inspect  and copy or make  extracts  from its books  showing  the  registered
holders of Warrants. All Warrants issued

4

upon the transfer or  assignment  of this Warrant will be dated the same date as
this Warrant,  and all rights of the holder  thereof shall be identical to those
of the Holder.

5.  Loss,   etc.,   of  Warrant.   Upon  receipt  of  evidence
satisfactory  to the Company of the loss,  theft,  destruction  or mutilation of
this Warrant, and of indemnity reasonably  satisfactory to the Company, if lost,
stolen or destroyed,  and upon surrender and  cancellation  of this Warrant,  if
mutilated,  the Company shall execute and deliver to the Holder a new Warrant of
like date, tenor and denomination.

6.       Investment Intent.

(a) The Holder represents,  by accepting this Warrant, that it
understands  that this Warrant and any  securities  obtainable  upon exercise of
this Warrant have not been registered for sale under Federal or state securities
laws and are  being  offered  and  sold to the  Holder  pursuant  to one or more
exemptions  from the  registration  requirements  of such  securities  laws. The
Holder is an "accredited  investor" within the meaning of Regulation D under the
Securities  Act of 1933, as amended (the "Act").  In the absence of an effective
registration of such securities or an exemption therefrom,  any certificates for
such  securities  shall bear the legend set forth on the first page hereof.  The
Holder understands that it must bear the economic risk of its investment in this
Warrant and any  securities  obtainable  upon  exercise  of this  Warrant for an
indefinite  period of time,  as this Warrant and such  securities  have not been
registered  under Federal or state  securities laws and therefore cannot be sold
unless  subsequently  registered under such laws,  unless as exemption from such
registration is available.

(b) The Holder,  by his acceptance of its Warrant,  represents
to the Company that it is acquiring this Warrant and will acquire any securities
obtainable  upon exercise of this Warrant for its own account for investment and
not with a view to, or for sale in connection with, any distribution  thereof in
violation  of the  Act.  The  Holder  agrees  that  this  Warrant  and any  such
securities will not be sold or otherwise  transferred  unless (i) a registration
statement  with  respect to such  transfer  is  effective  under the Act and any
applicable  state securities laws or (ii) such sale or transfer is made pursuant
to one or more exemptions from the Act.

7.  Status of Holder.  This  Warrant  does not confer upon the
Holder any right to vote or to consent to or receive  notice as a stockholder of
the Company, as such, in respect of any matters whatsoever,  or any other rights
or liabilities as a stockholder, prior to the exercise hereof.

8.  Notices.  No  notice  or other  communication  under  this
Warrant shall be effective unless, but any notice or other  communication  shall
be  effective  and shall be deemed to have been given if, the same is in writing
and is mailed by first-class mail, postage prepaid, addressed to:

(a)  the   Company   at  155   Fortune   Boulevard,   Milford,
Massachusetts,  01757  Attention:  E. Andrews  Grinstead,  III, or such
other  address as the Company has  designated in writing to the Holder;
or

(b) the  Holder  at  [___________________________________]  or
such  other  address as the  Holder  has  designated  in writing to the
Company.

5

9.  Headings.  The headings of this Warrant have been inserted
as a matter of convenience and shall not affect the construction hereof.

10.  Applicable  Law.  This  Warrant  shall be governed by and
construed  in  accordance  with  the law of the  Commonwealth  of  Massachusetts
without giving effect to principles of conflicts of law thereof.

6

IN WITNESS WHEREOF, E. Andrews Grinstead,  III, acting for and
on behalf of the  Company,  has executed  this Warrant and caused the  Company's
corporate seal to be hereunto affixed and attested by its Secretary or Assistant
Secretary as of December __, 1999.

HYBRIDON, INC.

By:____________________________________
Name:  E. Andrews Grinstead, III
Title: President and Chief Executive
Officer

ATTEST:

--------------------------------
Secretary or Assistant Secretary

[Corporate Seal]

7

SUBSCRIPTION

The undersigned, ____________________________, pursuant to the
provisions  of the  foregoing  Warrant,  hereby  elects to  exercise  the within
Warrant to the extent of purchasing _____________________ shares of Common Stock
of Hybridon,  Inc.  thereunder and hereby makes payment of  $_______________  by
certified or official bank check in payment of the exercise price therefor.

Dated:_______________                       Signature:__________________________

Address:_______________________________

CASHLESS EXERCISE

The undersigned, ____________________________, pursuant to the
provisions  of the  foregoing  Warrant,  hereby  elects to  exchange  the within
Warrant for ______________ shares of Common Stock of Hybridon,  Inc. pursuant to
the cashless exercise provisions of the Warrant. The undersigned hereby confirms
the representations and warranties made by it in the Warrant.

Dated:_______________                       Signature:__________________________

Address:_______________________________

8

ASSIGNMENT

FOR  VALUE  RECEIVED   _______________________________________
hereby sells,  assigns and transfers unto  _____________________________________
the foregoing  Warrant and all rights  evidenced  thereby,  and does irrevocably
constitute and appoint _____________________________, attorney, to transfer said
Warrant on the books of Hybridon, Inc.

Dated:_______________                      Signature:___________________________

Address:______________________________

PARTIAL ASSIGNMENT

FOR VALUE RECEIVED  __________________________  hereby assigns
and transfers unto  _________________________  the right to purchase  __________
shares of the Common Stock, no par value per share, of Hybridon, Inc. covered by
the foregoing  Warrant,  and a proportionate part of said Warrant and the rights
evidenced    thereby,    and   does    irrevocably    constitute   and   appoint
__________________________,  attorney,  to transfer that part of said Warrant on
the books of Hybridon, Inc.

Dated:_______________                       Signature:__________________________

Address:_____________________________

9

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No.___
Note due 2002

$___________
[DATE OF ISSUANCE]

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay to  ___________________________  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future  Operating  Indebtedness

of the  Company.  The  Notes  are  secured  by the  Collateral  pursuant  to the
Subscription  Agreement,  and the security interests granted therein are subject
to any prior security  interest in the Collateral  granted by the Company except
as modified by the Intercreditor Agreement.

SECTION 1.             Interest.

The  Company  will pay  interest  semi-annually  in arrears on
April 1 and October 1 of each year (each an "Interest  Payment Date"), or if any
such day is not a  Business  Day,  on the next  succeeding  Business  Day to the
registered  Holder hereof as of the preceding  March 15 or September 15 (each, a
"Record Date").  Interest on this Note will accrue from the most recent Interest
Payment  Date to which  interest has been paid or, if no interest has been paid,
from the date of its  issuance  set forth  above;  provided  that if there is no
existing  Default in the payment of interest,  and if this Note is authenticated
between a Record Date, and the next succeeding  Interest Payment Date,  interest
shall accrue from such next succeeding  Interest  Payment Date. The Company may,
with  respect  to each  Interest  Payment  Date,  at its  option and in its sole
discretion,  in  lieu of  payment  of  interest  on the  Notes  in  cash,  issue
additional  Notes ("Interest  Notes") in an aggregate  principal amount equal to
the amount of interest  not paid in cash on such  Interest  Payment  Date.  Each
issuance of Interest  Notes in lieu of the payment of cash interest on the Notes
shall be made pro rata with respect to the outstanding Notes; provided, however,
that the Company may at its option pay cash in lieu of issuing Interest Notes in
any denomination of less than $1,000.  Interest will be computed on the basis of
a 360-day year comprised of twelve 30-day months.

SECTION 2.             Prepayment.

(a) This Note  (including  interest  accrued on the  principal
hereof) may be prepaid by the Company, at any time, in whole or in part, without
penalty or premium except as provided in Subsection 2(b).

(b) If this Note (or any  portion  hereof)  is  prepaid by the
Company,  then the Company shall simultaneously issue to the Holder hereof Class
E Warrants  ("Warrants") to purchase a number of shares of Common Stock equal to
the number of shares of Common Stock then issuable upon  conversion of this Note
(or the  prepaid  portion  hereof,  if prepaid  in part).  Such  Warrants  shall
initially be exercisable at $.60 per share of Common Stock and shall be governed
by a  Warrant  Agreement  by and  among  the  Company,  ChaseMellon  Shareholder
Services, L.L.C. and the Secured Party in substantially the form attached to the
Subscription Agreement as Exhibit B (the "Warrant Agreement").

SECTION 3.        Conversion

(a) Conversion. The Holder may elect, at any time prior to the
Maturity  Date,  to convert  this Note and all  accrued  interest  hereon into a
number of shares of Common Stock equal to Liquidation  Amount (as defined below)
divided  by  the  then  current   Conversion  Price  (as  defined  below).   The
"Liquidation  Amount"  shall be the  aggregate  principal  amount  of,  plus any
accrued  but  unpaid  interest  on,  this Note.  The  "Conversion  Price"  shall
initially be $0.60,  subject to adjustment as provided  below,  representing  an
initial  conversion  rate (subject to

adjustment) of 1,666-2/3 shares of Common Stock per $1,000 of Liquidation Amount
(the "Conversion Rate").

(b) Conversion  Procedures.  (i) Any Holder of a Note desiring
to  convert  such  Note  into  Common  Stock  shall  surrender  such Note at the
Company's  principal  executive  office,  accompanied  by proper  instruments of
transfer to the Company or in blank,  accompanied by irrevocable  written notice
to the Company  that the Holder  elects so to convert  such Note (the "Notice of
Conversion")  and  specifying  the  name or  names  (with  address)  in  which a
certificate or certificates evidencing shares of Common Stock are to be issued.

(ii) The Company  need not deem a Notice of  Conversion  to be
received unless the Holder complies with all the provisions  hereof. The Company
will make a notation of the date that a Notice of Conversion is received,  which
date of receipt shall be deemed to be the date of receipt for purposes hereof.

(iii) The Company  shall,  as soon as  practicable  after such
deposit of any Note  accompanied by a Notice of Conversion  and compliance  with
any other conditions herein  contained,  deliver to the person for whose account
such Note was so  surrendered,  or to the nominee or  nominees  of such  person,
certificates  evidencing the number of full shares of Common Stock to which such
person shall be entitled as aforesaid, subject to Section 4.

(iv) Subject to the  following  provisions  of this  Paragraph
3(b)(iv),  such  conversion  shall be deemed to have been made as of the date of
such surrender of the Note to be converted,  and the person or persons  entitled
to receive the Common Stock  deliverable  upon  conversion of such Note shall be
treated for all purposes as the record holder or holders of such Common Stock on
such date and the Note  shall no longer be  deemed  outstanding  and all  rights
whatsoever in respect thereof  (including the right to receive interest thereon)
shall terminate  except the right to receive the number of full shares of Common
Stock to which such person shall be entitled hereunder;  provided, however, that
the Company  shall not be required to convert any Note while the stock  transfer
books of the Company are closed for any purpose, but the surrender of a Note for
conversion  during  any  period  while  such  books are so closed  shall  become
effective for conversion  immediately upon the reopening of such books as if the
surrender had been made on the date of such reopening,  and the conversion shall
be at the  Conversion  Rate in effect on such date  applied  to the  Liquidation
Amount calculated through such date of reopening.

(c) Adjustments to Conversion  Price.  (i) In case the Company
shall hereafter (A) pay a dividend or make a distribution on its Common Stock in
shares of Common Stock,  (B) subdivide  its  outstanding  shares of Common Stock
into a greater number of shares or (C) combine its outstanding  shares of Common
Stock into a smaller number of shares (each of (A) through (C) an "Action"), the
Conversion  Price shall be adjusted to equal the product of the Conversion Price
in  effect  immediately  prior to such  Action  multiplied  by a  fraction,  the
numerator  of which  shall be the number of shares of Common  Stock  outstanding
immediately  prior to such  Action  and the  denominator  of which  shall be the
number of shares of Common Stock outstanding  immediately following such Action.
An  adjustment  made  pursuant to this  Subsection  3(b) shall become  effective
immediately  after the record date in the case of a

dividend  or  distribution  and shall  become  effective  immediately  after the
effective date in the case of a subdivision, combination or reclassification.

(ii)  In  case   the   Company   shall   hereafter   issue  by
reclassification  of its Common Stock any shares of capital stock of the Company
(a  "Reclassification"),  provision shall be made so that, immediately following
such Reclassification, the Notes shall be convertible into the kind and quantity
of  securities  to which the  Holders  of such Notes  would  have been  entitled
pursuant  to such  Reclassification,  had  such  Holders  converted  such  Notes
immediately prior to such Reclassification.

(d) Reservation of Shares;  Transfer  Taxes;  Etc. The Company
shall  at all  times  reserve  and keep  available,  out of its  authorized  and
unissued  shares of Common  Stock,  solely  for the  purpose  of  effecting  the
conversion  of the  Notes,  such  number of shares of its  Common  Stock free of
preemptive  rights as shall be sufficient to effect the  conversion of all Notes
from time to time outstanding.  The Company shall use its best efforts from time
to time, in accordance  with the laws of the State of Delaware,  to increase the
authorized  number of shares of Common Stock if at any time the number of shares
of Common Stock not outstanding shall not be sufficient to permit the conversion
of all the then-outstanding Notes.

The Company  shall pay any and all issue or other taxes (other
than  income  taxes)  that may be payable in respect of any issue or delivery of
shares of Common  Stock on  conversion  of the  Notes.  The  Company  shall not,
however,  be  required  to pay any tax which may be  payable  in  respect of any
transfer  involved in the issue or delivery of Common Stock (or other securities
or  assets)  in a name  other  than that in which the  Notes so  converted  were
registered,  and no such issue or  delivery  shall be made  unless and until the
person  requesting  such issue has paid to the Company the amount of such tax or
has  established,  to the  satisfaction  of the Company,  that such tax has been
paid.

(e) Other Changes in Conversion Rate. The Company from time to
time may  increase the  Conversion  Rate by any amount for any period of time if
the period is at least 20 days and if the  increase  is  irrevocable  during the
period. Whenever the Conversion Rate is so increased,  the Company shall mail to
the  Holder of record  of this  Note a notice of the  increase  at least 15 days
before the date the  increased  Conversion  Rate takes  effect,  and such notice
shall state the increased Conversion Rate and the period it will be in effect.

The Company may make such increases in the Conversion Rate, in
addition  to those  required  or  allowed  by this  paragraph  (e),  as shall be
determined  by it, as evidenced by a resolution of the Board of Directors of the
Company, to be advisable in order to avoid or diminish any income tax to holders
of Common Stock resulting from any dividend or distribution of stock or issuance
of rights or  warrants  to  purchase  or  subscribe  for stock or from any event
treated as such for income tax purposes.

SECTION 4.             Fractional Shares.

No fractional shares or scrip  representing  fractional shares
of Common Stock shall be issued upon  conversion  of this Note. If more than one
certificate  evidencing Notes shall be surrendered for conversion at one time by
the same Holder,  the number of full shares  issuable

upon  conversion  thereof  shall  be  computed  on the  basis  of the  aggregate
Liquidation Amount of the Notes so surrendered.  Instead of any fractional share
of Common Stock which would  otherwise be issuable upon  conversion of this Note
(or of such  aggregate  number of Notes),  the  Company  may elect,  in its sole
discretion,  independently  for each  Holder,  whether  such number of shares of
Common  Stock will be rounded to the  nearest  whole share (with a .5 of a share
rounded  upward) or  whether  such  Holder  will be given  cash,  in lieu of any
fractional  share,  in an amount  equal to the same  fraction of the  Conversion
Price as of the close of business on the day of conversion.

SECTION 5.             Events of Default Defined.

The  following  shall each  constitute  an "Event of  Default"
hereunder:

(a) the  failure  of the  Company  to make any  payment of (i)
principal of this Note when due and payable and such failure shall  continue for
five (5) or more days;  and (ii)  interest on this Note when due and payable and
such failure shall continue for thirty (30) or more days;

(b) the  failure of the  Company  to  observe  or perform  any
covenant in this Note or in the Subscription  Agreement,  and such failure shall
have  continued  unremedied for a period of sixty (60) days after written notice
as provided in the last paragraph of this Section 5;

(c) a default  occurs (after  giving effect to any  applicable
grace periods or any extension of any maturity  date) in the payment when due of
principal of, or an acceleration  of, any indebtedness for money borrowed by the
Company or any of its  Subsidiaries  (other than an Unrestricted  Subsidiary (as
defined  below) which is not a  Significant  Subsidiary  (as defined  below) and
provided there is no recourse  against the Company or any other  Subsidiary with
respect to the obligations of such Unrestricted  Subsidiary  arising as a result
of such default) in excess of $2 million,  individually or in the aggregate,  if
such  indebtedness  is not  discharged,  or such  acceleration  is not annulled,
within 30 days after  written  notice as provided in the last  paragraph of this
Section 5;

(d)  the  Company  or  any of  its  Significant  Subsidiaries,
pursuant to or within the meaning of any Bankruptcy Law:

(i)      commences a voluntary case,

(ii)     consents  to the  entry of an order  for  relief
against it in an involuntary case,

(iii)    consents to the appointment of a Custodian of it
or  for  all or  substantially  all of  its  property,  and  such
Custodian is not discharged within 30 days,

(iv)     makes a general  assignment  for the  benefit of
its creditors, or

(v)      admits in writing that it is generally unable to
pay its debts as the same become due;

(e) a court  of  competent  jurisdiction  enters  an  order or
decree under any Bankruptcy Law that:

(i)      is for relief in any  involuntary  case  against
the Company or any Significant Subsidiary,

(ii)     appoints  a  Custodian  of  the  Company  or any
Significant  Subsidiary  or for all or  substantially  all of the
property of the Company or any Significant Subsidiary, or

(iii)    orders  the  liquidation  of the  Company or any
Significant  Subsidiary,  and, in each case,  the order or decree
remains unstayed and in effect for 60 consecutive days.

The term  "Bankruptcy  Law" means Title 11 of the U.S. Code or
any similar  federal,  foreign or state law for the relief of debtors.  The term
"Custodian"  means any  receiver,  trustee,  assignee,  liquidator,  examiner or
similar official under any Bankruptcy Law. The term "Significant Subsidiary" has
the same meaning as significant  subsidiary  has under  Regulation S-X under the
Securities Act as in effect on the date hereof.  "Unrestricted Subsidiary" means
any Subsidiary of the Company which (i) is not wholly-owned by the Company, (ii)
is  designated  as an  Unrestricted  Subsidiary by the Board of Directors of the
Company  and  (iii)  at the  time  of any  investment  by the  Company  in  such
Subsidiary,  in the aggregate  holds or comprises less than 20% of the Company's
assets  as  shown  on the  Company's  consolidated  balance  sheet  prepared  in
accordance with generally accepted accounting principles consistently applied as
at the time of such investment.

(f) the failure of the Company to maintain, as of the last day
of any  calendar  month,  consolidated  cash on hand (and cash  equivalents  and
marketable securties) of at least $1.5 million.

A Default  under  Subsection  (b),  (c) or (f) of this Section 5 shall not be an
Event of Default  until (i) the Secured Party shall have notified the Company of
the  Default and (ii) the  Company  shall have failed to cure the Default  under
such  Subsection  (b) within 60 days after  receipt  of the  notice,  under such
Subsection  (c) within 10 days after  receipt of the notice or under  Subsection
(f) within 30 days after receipt of the notice. Any such notice must (x) specify
the  Default,  (y) demand that it be remedied and (z) state that the notice is a
"Notice of Default."

SECTION 6.        Remedies upon Event of Default.

Except as limited by the Intercreditor Agreement:

(a) If an Event  of  Default  occurs  and is  continuing,  the
Secured Party (by notice to the Company) may declare the unpaid principal of and
accrued  interest  on all the Notes then  outstanding  to be due and payable (an
"Acceleration").  Upon any such declaration, such principal and accrued interest
shall  be due  and  payable  immediately.  The  Secured  Party  may  rescind  an
acceleration  and its  consequences if (a) the Company has paid a sum sufficient
to pay (i) all  overdue  interest  on all Notes  then  outstanding  and (ii) the
principal of the Notes then outstanding  which have become due otherwise than by
such declaration of acceleration and accrued interest thereon at a rate borne by
the Notes and (b) the rescission  would not conflict

with any  judgment  or decree and if all  existing  Events of Default  have been
cured or waived  except  nonpayment of principal or interest that has become due
solely because of  acceleration.  No such rescission shall effect any subsequent
Default or impair any right consequent thereto.

(b) The Secured  Party may waive an existing  Default or Event
of Default and its consequences.  Upon any such waiver, such Default shall cease
to exist and any Event of Default arising therefrom shall be deemed to have been
cured for every purpose of this Note and the Subscription Agreement; but no such
waiver  shall  extend to any  subsequent  or other  Default  or impair any right
consequent thereon.

(c) If the  Company  defaults  in a payment of interest on the
Notes, then, in lieu of this Note's ordinary 8% interest,  the Company shall pay
defaulted interest at a rate of 12% (or, during the first six months immediately
following any  Acceleration,  16%, and  thereafter  24%) per annum.  The Company
shall pay the defaulted interest to the Holders of the Notes on a special record
date.  The Company  shall fix or cause to be fixed any such special  record date
and payment date,  which  specified  record date shall not be fewer than 10 days
prior to the payment date for such defaulted  interest,  and shall promptly mail
or cause to be mailed to each Holder a notice  that  states the  special  record
date, the payment date and the amount of defaulted interest to be paid.

(d) Upon the occurrence and during the continuance of an Event
of  Default,  the  Secured  Party may, at its  election,  without  notice of its
election and without  demand,  take any action  permitted by law,  including the
exercise of any rights accorded a secured creditor under the Uniform  Commercial
Code as in effect in the Commonwealth of Massachusetts at such time.

(e) To the extent  permitted  by law,  the  remedies  provided
herein shall be exclusive of any other remedies now or hereafter existing at law
or in equity or by statute or otherwise.

(f) In any suit for the  enforcement  of any  right or  remedy
under this Note or the  Subscription  Agreement,  a court in its  discretion may
require the filing by any party  litigant in the suit of an  undertaking  to pay
the costs of the suit,  and the court in its  discretion  may assess  reasonable
costs,  including reasonable  attorneys' fees, against any party litigant in the
suit,  having due regard to the merits and good faith of the claims or  defenses
made by the party litigant.

SECTION 7              Note Register.

(a) The Company shall keep at its principal executive office a
register  (herein  sometimes  referred  to as the  "Note  Register"),  in which,
subject to such reasonable  regulations as it may prescribe,  but at its expense
(other  than  transfer  taxes,  if  any),  the  Company  shall  provide  for the
registration and transfer of this Note.

(b) Whenever this Note shall be  surrendered  at the principal
executive  office of the Company  for  transfer or  exchange,  accompanied  by a
written  instrument of transfer in form  reasonably  satisfactory to the Company
duly executed by the Holder  hereof or his attorney duly  authorized in writing,
and,  subject to compliance with applicable  securities  laws, the Company shall
execute  and  deliver  in  exchange  therefor  a new  Note or  Notes,  as may be

requested by such Holder,  in the same  aggregate  unpaid  principal  amount and
payable  on the  same  date as the  principal  amount  of the  Note or  Notes so
surrendered;  each such new Note shall be dated as of the date to which interest
has been paid on the unpaid principal amount of the Note or Notes so surrendered
and shall be in such  principal  amount and  registered in such name or names as
such Holder may designate in writing.

(c)  Upon  receipt  by  the  Company  of  evidence  reasonably
satisfactory  to it of the loss,  theft,  destruction or mutilation of this Note
and of indemnity or bond reasonably  satisfactory to it, and upon  reimbursement
to the Company of all reasonable expenses incidental thereto, and upon surrender
and  cancellation of this Note (in case of mutilation) the Company will make and
deliver  in lieu of this  Note a new Note of like  tenor  and  unpaid  principal
amount  and dated as of the date to which  interest  has been paid on the unpaid
principal  amount  of this  Note in lieu of  which  such  new  Note is made  and
delivered.

SECTION 8              Miscellaneous.

(a) Amendments and Waivers. The Secured Party on behalf of the
Holders of the Notes may waive or otherwise  consent to the  amendment of any of
the provisions hereof,  provided that no such waiver or amendment may reduce the
principal  amount  of or  interest  on any of the  Notes or  change  the  stated
maturity of the  principal  of this Note,  without the consent of each holder of
any Note affected thereby.

(b)  Restrictions  on  Transferability.  In  addition  to  the
restrictions set forth in the Subscription Agreement, the securities represented
by this Note have been  acquired  for  investment  and have not been  registered
under the  Securities  Act of 1933, as amended,  or the  securities  laws of any
state or other jurisdiction.  Without such registration, such securities may not
be sold,  pledged,  hypothecated  or otherwise  transferred,  except pursuant to
exemptions from the Securities Act of 1933, as amended,  and the securities laws
of any state or other jurisdiction.

(c) Forbearance  from Suit. No holder of Notes shall institute
any suit or  proceeding  for the  enforcement  of the  payment of  principal  or
interest unless the Secured Party joins in such suit or proceeding.

(d)  No  Recourse  Against  Others.  No  directors,   officer,
employee,  incorporator or stockholder of the Company,  as such,  shall have any
liability for any  obligations of the Company under this Note, the  Subscription
Agreement  or for any claim  based  on, in  respect  of, or by reason  of,  such
obligations  or their  creation.  The Holder of this Note by accepting this Note
waives and releases all such  liability.  The waiver and release are part of the
consideration for the issuance of this Note.

(e)  Subordination.  The Holder by accepting  this Note agrees
that the payment (by set-off or  otherwise)  of principal of and interest on the
Notes is  subordinated  in right of  payment,  to the  extent  and in the manner
provided in Section 9 of the  Subscription  Agreement,  to the prior  payment in
full of all  obligations  in respect of Operating  Indebtedness  of the Company,
whether  outstanding  on the date of the  Subscription  Agreement or  thereafter
incurred.

(f)   Denominations.   This  Note  is   issuable   in  minimum
denominations  of $1,000 and  integral  multiples  of $1,000 in excess  thereof,
except as otherwise provided in Section 1 hereof.

(g)  Governing  Law.  This  Note  shall be  governed  by,  and
construed in accordance with, the laws of the State of Massachusetts,  excluding
the body of law  relating to conflict of laws.  Notwithstanding  anything to the
contrary  contained  herein,  in no event  may the  effective  rate of  interest
collected or received by the Holder exceed that which may be charged,  collected
or received by the Holder under applicable law.

(h)  Interpretation.  If any term or  provision  of this  Note
shall be held invalid, illegal or unenforceable, the validity of all other terms
and provisions hereof shall in no way be affected thereby.

(i)  Successors  and Assigns.  This Note shall be binding upon
the Company and its successors and assigns and shall inure to the benefit of the
Holder and its successors and registered assigns.

(j) Notices. All notices, requests, consents and demands shall
be made in writing and shall be mailed postage prepaid, or delivered by hand, to
the Company or to the Holder  thereof at their  respective  addresses  set forth
below or to such other address as may be furnished in writing to the other party
hereto:

If to the Holder:    At the address shown on Schedule A attached hereto

If to the Company:   Hybridon, Inc.
155 Fortune Boulevard
Milford, Massachusetts 01757
Attention: E. Andrews Grinstead, III

(k) Saturdays,  Sundays, Holidays. If any date that may at any
time be  specified  in this  Note as a date for the  making  of any  payment  of
principal or interest under this Note shall fall on Saturday, Sunday or on a day
which in New York or Massachusetts or California shall be a legal holiday,  then
the date for the making of that payment shall be the next  subsequent  day which
is not a Saturday, Sunday or legal holiday.

(l) Subscription Agreement.  This Note is subject to the terms
contained in the Subscription  Agreement and the registered  Holder of this Note
is  entitled  to the  benefits  of such  Subscription  Agreement  to the  extent
provided therein.

(m) No Adverse  Interpretation of Other Agreements.  This Note
and the  Subscription  Agreement  may not be used  to  interpret  another  note,
indenture, loan or debt agreement of the Company or a Subsidiary. Any such note,
indenture,  loan or debt agreement may not be used to interpret this Note or the
Subscription Agreement.

IN WITNESS  WHEREOF,  this Note due 2002 No. __ has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:___________________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 1
Note due 2002

$60,000

December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to Essam A. J. Alamdar (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 1 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:__________________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Essam A. J. Alamdar                                  Riyadh-11431
Saudi Arabia

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 2
Note due 2002

$25,000

December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay to  Mansour S. M. A.  Al-Sharif  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 2 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Mansour S. M. A. Al-Sharif

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 3
Note due 2002

$75,000

December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to  pay  to  H.K.  Properties  Limited  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 3 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:______________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

H.K. Properties Limited

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 4
Note due 2002

$75,000

December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby promises to pay to Kincroft  Limited (the "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 4 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:______________________
Name:
Title:

SCHEDULE A

Name of Holder                                    Address of Holder

Kincroft Limited                                  Ridgeway House, Ridgeway St.
Douglas, Isle of Man

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 5
Note due 2002

$100,000

December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby promises to pay to Fouad M. O. Tawfig (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 5 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:________________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Fouad M. O. Tawfig

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 6
Note due 2002

$50,000

December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby promises to pay to Fouad M. O. Tawfig (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 6 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_______________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Fouad M. O. Tawfig

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 7
Note due 2002

$20,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby promises to pay to Fouad M. O. Tawfig (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 7 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:______________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Fouad M. O. Tawfig

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 8
Note due 2002

$120,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby  promises to pay to Seif  Foundation (the "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 8 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_________________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Seif Foundation

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 9
Note due 2002

$250,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay  to  Bajrai  Int'l  Group  Ltd.  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN  WITNESS  WHEREOF,  this Note due 2002 No. 9 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_______________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Bajrai Int'l Group Ltd.

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 10
Note due 2002

$400,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby promises to pay to Darier, Hentsch & Cie (the "Holder"),
or  registered  assigns,  the  principal  sum set forth above,  with accrued but
unpaid  interest  thereon at a rate equal to eight  percent  (8%) per annum,  on
November 30, 2002 (the "Maturity Date").  Payment shall be made at such place as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 10 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:__________________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Darier, Hentsch & Cie

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 11
Note due 2002

$150,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby promises to pay to Solter Corporation (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 11 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:________________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Solter Corporation

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 12
Note due 2002

$75,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay to  Malek  Salam  and  Oussama  Salam
(jointly,  the  "Holder"),  or registered  assigns,  the principal sum set forth
above, with accrued but unpaid interest thereon at a rate equal to eight percent
(8%) per annum,  on November 30, 2002 (the  "Maturity  Date").  Payment shall be
made at such place as designated by the Company upon  surrender of this Note (as
defined  below),  and shall be in such coin or currency of the United  States of
America  as at the time of  payment  shall be legal  tender  for the  payment of
public  and  private  debts.  This  Note is one of a duly  authorized  issue  of
Hybridon,  Inc.  Notes due 2002  (individually  a "Note"  and  collectively  the
"Notes") issued pursuant to a Subscription Agreement which is available from the
Company (the "Subscription Agreement") and similar agreements. Capitalized terms
used herein without  definition have the respective  meanings specified therefor
in the  Subscription  Agreement.  The Notes  shall be  subordinated  in right of
payment to all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 12 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:__________________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Malek Salam and Oussama Salam

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 13
Note due 2002

$40,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby promises to pay to Mohamad Khaled Omari (the "Holder"),
or  registered  assigns,  the  principal  sum set forth above,  with accrued but
unpaid  interest  thereon at a rate equal to eight  percent  (8%) per annum,  on
November 30, 2002 (the "Maturity Date").  Payment shall be made at such place as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 13 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_______________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Mohamad Khaled Omari

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 14
Note due 2002

$300,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay  to  Khaled  M.  K.  Abdulghani  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 14 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Khaled M. K. Abdulghani

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 15
Note due 2002

$200,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to Sylvione Abu Haidar (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 15 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Sylvigne Abu Haidar

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 16
Note due 2002

$600,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises to pay to Nicris Limited (the  "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 16 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Nicris Limited

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 17
Note due 2002

$200,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises to pay to Clapham  Investments  Limited  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 17 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Clapham Investments Limited

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 18
Note due 2002

$60,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay to  Torben  Duer (the  "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 18 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Torben Duer

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 19
Note due 2002

$30,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby promises to pay to Nafez M. M. Al-Jindi (the "Holder"),
or  registered  assigns,  the  principal  sum set forth above,  with accrued but
unpaid  interest  thereon at a rate equal to eight  percent  (8%) per annum,  on
November 30, 2002 (the "Maturity Date").  Payment shall be made at such place as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 19 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Nafez M. M. Al-Jindi

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 20
Note due 2002

$300,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises to pay to Alain  Mallart (the  "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 20 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Alain Mallart

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 21
Note due 2002

$100,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to Imad Moustapha Mansour (the "Holder"),
or  registered  assigns,  the  principal  sum set forth above,  with accrued but
unpaid  interest  thereon at a rate equal to eight  percent  (8%) per annum,  on
November 30, 2002 (the "Maturity Date").  Payment shall be made at such place as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 21 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Imad Moustapha Mansour

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 22
Note due 2002

$100,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby  promises to pay to Oussama M. Salam (the "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 22 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Oussama M. Salam

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 23
Note due 2002

$100,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises to pay to LGT Bank in  Liechtenstein  AG (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 23 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

LGT Bank in Liechtenstein AG

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 24
Note due 2002

$60,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby promises to pay to  Investeringsselskabet  af 1/12 1997
Ap.S (the "Holder"),  or registered assigns,  the principal sum set forth above,
with accrued but unpaid  interest  thereon at a rate equal to eight percent (8%)
per annum, on November 30, 2002 (the "Maturity Date").  Payment shall be made at
such place as designated by the Company upon  surrender of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 24 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Investeringsselskabet af 1/12 1997 Ap.S

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 25
Note due 2002

$15,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to MicroTech Software A/S (the "Holder"),
or  registered  assigns,  the  principal  sum set forth above,  with accrued but
unpaid  interest  thereon at a rate equal to eight  percent  (8%) per annum,  on
November 30, 2002 (the "Maturity Date").  Payment shall be made at such place as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 25 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

MicroTech Software A/S

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 26
Note due 2002

$6,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby  promises to pay to JSP Holding Ap.S (the "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 26 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

JSP Holding Ap.S

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 27
Note due 2002

$50,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby  promises to pay to Motasim F. Hajaj (the "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 27 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Motasim F. Hajaj

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 28
Note due 2002

$250,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received,  hereby promises to pay to Mohamad A. Bajria (the "Holder"),  or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 28 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Mohamad A. Bajria

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 29
Note due 2002

$1,500,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay to E.  Andrews  Grinstead,  III  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 29 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

E. Andrews Grinstead, III

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 30
Note due 2002

$60,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to Essam A. J. Alamdar (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 30 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Essam A. J. Alamdar

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 31
Note due 2002

$[366,390]
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to  pay  to  Pillar  Investments  Ltd.  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 31 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Pillar Investments Ltd.                              28 Avenue de Messine
Paris, France 75008

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 32
Note due 2002

$250,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to Christopher S. Gaffney (the "Holder"),
or  registered  assigns,  the  principal  sum set forth above,  with accrued but
unpaid  interest  thereon at a rate equal to eight  percent  (8%) per annum,  on
November 30, 2002 (the "Maturity Date").  Payment shall be made at such place as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 32 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Christopher S. Gaffney                               3 Winthrop Street
West Newton, MA  02465

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 33
Note due 2002

$10,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to Jonathan P. Raymond (the "Holder"), or
registered  assigns,  the principal sum set forth above, with accrued but unpaid
interest  thereon at a rate equal to eight  percent (8%) per annum,  on November
30,  2002  (the  "Maturity  Date").  Payment  shall  be made at  such  place  as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 33 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Jonathan P. Raymond                                  133 Park St., #1207
Brookline, MA  02446

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 34
Note due 2002

$250,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to pay to  Forum  Capital  Markets  LLC (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 34 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Forum Capital Markets LLC                            53 Forest Ave.
Old Greenwich, CT  06870

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. 35
Note due 2002

$105,000
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value received, hereby promises to pay to Samuel Morrill Robbins (the "Holder"),
or  registered  assigns,  the  principal  sum set forth above,  with accrued but
unpaid  interest  thereon at a rate equal to eight  percent  (8%) per annum,  on
November 30, 2002 (the "Maturity Date").  Payment shall be made at such place as
designated by the Company upon  surrender of this Note (as defined  below),  and
shall be in such coin or currency of the United States of America as at the time
of payment  shall be legal  tender for the payment of public and private  debts.
This Note is one of a duly  authorized  issue of Hybridon,  Inc.  Notes due 2002
(individually  a "Note" and  collectively  the  "Notes")  issued  pursuant  to a
Subscription  Agreement  which is available from the Company (the  "Subscription
Agreement")  and  similar  agreements.  Capitalized  terms used  herein  without
definition have the respective  meanings  specified therefor in the Subscription
Agreement.  The Notes shall be  subordinated in right of payment to all existing
and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. 35 has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

Samuel Morrill Robbins                               300 Prince Street
West Newton, MA  02465

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. _
Note due 2002

$__________
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to  pay to  __________________________  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. __ has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. _
Note due 2002

$__________
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to  pay to  __________________________  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. __ has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

THE TERMS OF THIS NOTE ARE SUBJECT TO THE TERMS OF A SUBSCRIPTION  AGREEMENT AND
AN INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE AVAILABLE FROM HYBRIDON,  INC.
(THE  "COMPANY").  THE  SECURITIES  REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933,  AS AMENDED  (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD,
OFFERED  FOR SALE,  PLEDGED OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN THE
ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT WITH  RESPECT TO THE  SECURITIES
UNDER THE  SECURITIES  ACT OR AN  EXEMPTION  FROM THE  SECURITIES  ACT. ANY SUCH
TRANSFER MAY ALSO BE SUBJECT TO COMPLIANCE WITH APPLICABLE STATE SECURITIES LAWS
AND THE LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT) IS THE EXCLUSIVE
AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT TO CERTAIN  ACTIONS  HEREUNDER AND
UNDER THE SUBSCRIPTION AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION, MAY
TAKE  OR  FOREBEAR  FROM  TAKING  CERTAIN   ACTIONS   HEREUNDER  AND  UNDER  THE
SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

No. _
Note due 2002

$_________
December 13, 1999

Hybridon, Inc., a Delaware corporation,  (the "Company"),  for
value  received,  hereby  promises  to  pay to  __________________________  (the
"Holder"),  or  registered  assigns,  the  principal  sum set forth above,  with
accrued but unpaid  interest  thereon at a rate equal to eight  percent (8%) per
annum, on November 30, 2002 (the "Maturity Date"). Payment shall be made at such
place as  designated  by the  Company  upon  surrender  of this Note (as defined
below), and shall be in such coin or currency of the United States of America as
at the time of  payment  shall be legal  tender  for the  payment  of public and
private debts.  This Note is one of a duly  authorized  issue of Hybridon,  Inc.
Notes due 2002  (individually  a "Note" and  collectively  the  "Notes")  issued
pursuant to a  Subscription  Agreement  which is available from the Company (the
"Subscription Agreement") and similar agreements.  Capitalized terms used herein
without  definition  have the  respective  meanings  specified  therefor  in the
Subscription  Agreement.  The Notes shall be subordinated in right of payment to
all existing and future Operating Indebtedness

IN WITNESS  WHEREOF,  this Note due 2002 No. __ has been  executed  and
delivered on the date first above written by the duly authorized  representative
of the Company.

HYBRIDON, INC.

By:_____________________
Name:
Title:

SCHEDULE A

Name of Holder                                       Address of Holder

THE TERMS OF THE NOTES ISSUABLE UPON EXERCISE HEREOF ARE SUBJECT TO THE TERMS OF
A SUBSCRIPTION  AGREEMENT AND AN  INTERCREDITOR  AGREEMENT,  COPIES OF WHICH ARE
AVAILABLE FROM HYBRIDON, INC. (THE "COMPANY").  NEITHER THESE SECURITIES NOR THE
NOTES  ISSUABLE  UPON  EXERCISE  HEREOF  NOR  THE  COMMON  STOCK  ISSUABLE  UPON
CONVERSION  THEREOF HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 OR ANY
APPLICABLE  STATE  SECURITIES  LAWS.  THEY MAY NOT BE SOLD,  OFFERED  FOR  SALE,
PLEDGED  OR  HYPOTHECATED   OR  OTHERWISE   TRANSFERRED  IN  THE  ABSENCE  OF  A
REGISTRATION  STATEMENT IN EFFECT WITH RESPECT TO THE SECURITIES  UNDER SUCH ACT
OR AN EXEMPTION  THEREFROM.  ANY SUCH TRANSFER MAY ALSO BE SUBJECT TO APPLICABLE
STATE SECURITIES LAWS.

THE  SECURED  PARTY (AS  DEFINED  IN THE  SUBSCRIPTION  AGREEMENT)  SHALL BE THE
EXCLUSIVE  AGENT OF THE HOLDER OF THE NOTES  ISSUABLE UPON EXERCISE  HEREOF WITH
RESPECT  TO  CERTAIN  ACTIONS  UNDER  SUCH  NOTES  AND  UNDER  THE  SUBSCRIPTION
AGREEMENT; THE SECURED PARTY, IN ITS SOLE DISCRETION,  MAY TAKE OR FOREBEAR FROM
TAKING CERTAIN ACTIONS UNDER THE NOTES AND UNDER THE  SUBSCRIPTION  AGREEMENT ON
BEHALF OF THE HOLDERS OF NOTES.

HYBRIDON, INC.

Class G Warrant for the Purchase of

Notes due 2002

No. G-1                                                $387,100 principal amount
of Notes due 2002

FOR VALUE  RECEIVED,  HYBRIDON,  INC., a Delaware  corporation
(the "Company"), hereby certifies that Pillar Investments Ltd. or its registered
assigns (the "Holder") is entitled to purchase from the Company,  subject to the
provisions of this Warrant (the "Warrant"), at any time on or after December 13,
1999 (the "Initial Exercise Date"),  and prior to 5:00 P.M., New York City time,
on November  30,  2006 (the  "Termination  Date"),  three  hundred  eighty-seven
thousand  one  hundred  dollars  ($387,100)  principal  amount of Notes due 2002
("Notes") of the Company, at an exercise price of one hundred ten percent (110%)
of such Notes' principal amount, for an aggregate exercise price of four hundred
twenty-five  thousand  eight  hundred  ten  dollars  ($425,810)  (the  aggregate
purchase price payable for the Warrant Notes hereunder is hereinafter  sometimes
referred to as the "Aggregate Exercise Price").  The Notes deliverable upon such
exercise  are  hereinafter  sometimes  referred to as the  "Warrant  Notes." The
exercise  price per dollar  principal  amount of Notes (i.e.  one dollar and ten
cents ($1.10)) is hereinafter  sometimes referred to as the "Per Dollar Exercise
Price."

24

1.       Exercise of Warrant.

(a) This Warrant may be exercised in whole or in part,  at any
time by its Holder  commencing  on the  Initial  Exercise  Date and prior to the
Termination  Date by presentation  and surrender of this Warrant,  together with
the duly executed  subscription  form attached at the end hereof, at the address
set forth in Subsection  8(a) hereof,  together  with  payment,  by certified or
official bank check or wire transfer payable to the order of the Company, of the
Aggregate Exercise Price or the proportionate part thereof if exercised in part.

(b) If this  Warrant is  exercised  in part only,  the Company
shall, upon presentation of this Warrant upon such exercise, execute and deliver
(along with the  certificate  for the  Warrant  Notes  purchased)  a new Warrant
evidencing  the  rights of the  Holder  hereof to  purchase  the  balance of the
Warrant Notes purchasable hereunder upon the same terms and conditions as herein
set forth.  Upon proper  exercise of this Warrant,  the Company  promptly  shall
deliver the  Warrant  Notes to the Holder duly  legended  as  authorized  by the
subscription form.

(c) The  execution of this Warrant shall also  constitute  the
Holder's  agreement to be bound by the terms of a  Subscription  Agreement and a
Subordination  and Intercreditor  Agreement,  copies of which are available from
the Company.

2.  Reservation of Warrant Shares;  Fully Paid Shares;  Taxes.
The Company hereby  represents that it has, and until expiration of this Warrant
agrees that it shall,  reserve for  issuance or delivery  upon  exercise of this
Warrant, such number of Notes and such number of shares of Common Stock as shall
be required  for  issuance  and/or  delivery  upon  exercise of this Warrant and
conversion of the Notes  issuable upon exercise  hereof in full, and agrees that
all Warrant Notes and shares of Common Stock so issued and/or  delivered will be
validly  issued,  fully paid and  non-assessable,  and further agrees to pay all
taxes and charges that may be imposed upon such issuance and/or delivery.

3.       [Reserved.]

4.  Limited  Transferability.  This  Warrant  may not be sold,
transferred,  assigned or  hypothecated  by the Holder except in compliance with
the provisions of the Act and the applicable  state  securities "blue sky" laws,
and is so  transferable  only upon the books of the Company which it shall cause
to be maintained for such purpose.  The Company may treat the registered  Holder
of this  Warrant as he or it appears on the  Company's  books at any time as the
Holder for all purposes. The Company shall permit any Holder of a Warrant or his
duly authorized  attorney,  upon written request during ordinary business hours,
to inspect  and copy or make  extracts  from its books  showing  the  registered
holders of Warrants. All Warrants issued upon the transfer or assignment of this
Warrant  will be dated  the same  date as this  Warrant,  and all  rights of the
holder thereof shall be identical to those of the Holder.

5.  Loss,   etc.,   of  Warrant.   Upon  receipt  of  evidence
satisfactory  to the Company of the loss,  theft,  destruction  or mutilation of
this Warrant, and of indemnity reasonably  satisfactory to the Company, if lost,
stolen or destroyed,  and upon surrender and

2

cancellation  of this  Warrant,  if  mutilated,  the Company  shall  execute and
deliver to the Holder a new Warrant of like date, tenor and denomination.

6.       Investment Intent.

(a) The Holder represents,  by accepting this Warrant, that it
understands  that this Warrant and any  securities  obtainable  upon exercise of
this Warrant have not been registered for sale under Federal or state securities
laws and are  being  offered  and  sold to the  Holder  pursuant  to one or more
exemptions  from the  registration  requirements  of such  securities  laws. The
Holder is an "accredited  investor" within the meaning of Regulation D under the
Securities  Act of 1933, as amended (the "Act").  In the absence of an effective
registration of such securities or an exemption therefrom,  any certificates for
such securities shall bear the legend set forth on the subscription form hereof.
The Holder  understands that it must bear the economic risk of its investment in
this Warrant and any securities  obtainable upon exercise of this Warrant for an
indefinite  period of time,  as this Warrant and such  securities  have not been
registered  under Federal or state  securities laws and therefore cannot be sold
unless  subsequently  registered under such laws,  unless as exemption from such
registration is available.

(b) The Holder, by his acceptance of this Warrant,  represents
to the Company  that it is  acquiring  this  Warrant and will  acquire any Notes
obtainable  upon  exercise  of this  Warrant  and any  shares  of  Common  Stock
obtainable  upon conversion of such Notes for its own account for investment and
not with a view to, or for sale in connection with, any distribution  thereof in
violation of the Act. The Holder agrees that this Warrant and any such Notes and
Common Stock will not be sold or otherwise transferred unless (i) a registration
statement  with  respect to such  transfer  is  effective  under the Act and any
applicable  state securities laws or (ii) such sale or transfer is made pursuant
to one or more exemptions from the Act.

7.  Status of Holder.  This  Warrant  does not confer upon the
Holder any right to vote or to consent to or receive  notice as a stockholder of
the Company, as such, in respect of any matters whatsoever,  or any other rights
or liabilities as a stockholder, prior to the exercise hereof.

8.  Notices.  No  notice  or other  communication  under  this
Warrant shall be effective unless, but any notice or other  communication  shall
be  effective  and shall be deemed to have been given if, the same is in writing
and is mailed by first-class mail, postage prepaid, addressed to:

(a)  the   Company   at  155   Fortune   Boulevard,   Milford,
Massachusetts,  01757  Attention:  E. Andrews  Grinstead,  III, or such
other  address as the Company has  designated in writing to the Holder;
or

(b) the  Holder  at  Pillar  Investments  Ltd.,  28  Avenue de
Messine,  Paris,  France 75008 or such other  address as the Holder has
designated in writing to the Company.

9.  Headings.  The headings of this Warrant have been inserted
as a matter of convenience and shall not affect the construction hereof.

3

10.  Applicable  Law.  This  Warrant  shall be governed by and
construed  in  accordance  with  the law of the  Commonwealth  of  Massachusetts
without giving effect to principles of conflicts of law thereof.

4

IN WITNESS WHEREOF, E. Andrews Grinstead,  III, acting for and
on behalf of the  Company,  has executed  this Warrant and caused the  Company's
corporate seal to be hereunto affixed and attested by its Secretary or Assistant
Secretary as of December __, 1999.

HYBRIDON, INC.

By:____________________________________
Name:  E. Andrews Grinstead, III
Title: President and Chief Executive
Officer

ATTEST:

--------------------------------
Secretary or Assistant Secretary

[Corporate Seal]

5

SUBSCRIPTION

The undersigned, ____________________________, pursuant to the
provisions  of the  foregoing  Warrant,  hereby  elects to  exercise  the within
Warrant to the extent of purchasing $_________________ principal amount of Notes
due 2002 thereunder and hereby makes payment of $_______________ by certified or
official bank check in payment of the exercise price  therefor.  The undersigned
further consents to the placement of the following legends on such Notes:

THE  TERMS OF THIS  NOTE ARE  SUBJECT  TO THE  TERMS OF A  SUBSCRIPTION
AGREEMENT AND AN INTERCREDITOR AGREEMENT, COPIES OF WHICH ARE AVAILABLE
FROM HYBRIDON, INC. (THE "COMPANY"). THE SECURITIES REPRESENTED BY THIS
NOTE HAVE NOT BEEN REGISTERED UNDER THE UNITED STATES SECURITIES ACT OF
1933,  AS AMENDED  (THE  "SECURITIES  ACT"),  OR ANY  APPLICABLE  STATE
SECURITIES  LAWS,  AND MAY NOT BE SOLD,  OFFERED  FOR SALE,  PLEDGED OR
HYPOTHECATED OR OTHERWISE  TRANSFERRED IN THE ABSENCE OF A REGISTRATION
STATEMENT IN EFFECT WITH RESPECT TO THE SECURITIES UNDER THE SECURITIES
ACT OR AN EXEMPTION FROM THE SECURITIES ACT. ANY SUCH TRANSFER MAY ALSO
BE SUBJECT TO COMPLIANCE WITH APPLICABLE  STATE SECURITIES LAWS AND THE
LAWS OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED  PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT)  IS THE
EXCLUSIVE  AGENT OF THE  HOLDER OF THIS NOTE WITH  RESPECT  TO  CERTAIN
ACTIONS  HEREUNDER AND UNDER THE  SUBSCRIPTION  AGREEMENT;  THE SECURED
PARTY, IN ITS SOLE DISCRETION, MAY TAKE OR FOREBEAR FROM TAKING CERTAIN
ACTIONS HEREUNDER AND UNDER THE SUBSCRIPTION AGREEMENT ON BEHALF OF THE
HOLDERS OF NOTES.

Dated:_______________                   Signature:_____________________________

Address:_______________________________

6

ASSIGNMENT

FOR  VALUE  RECEIVED   _______________________________________
hereby sells,  assigns and transfers unto  _____________________________________
the foregoing  Warrant and all rights  evidenced  thereby,  and does irrevocably
constitute and appoint _____________________________, attorney, to transfer said
Warrant on the books of Hybridon, Inc.

Dated:_______________                   Signature:_____________________________

Address:______________________________

PARTIAL ASSIGNMENT

FOR VALUE RECEIVED  __________________________  hereby assigns
and transfers unto  _________________________  the right to purchase $__________
principal  amount of Notes due 2002 of Hybridon,  Inc.  covered by the foregoing
Warrant,  and a  proportionate  part of said  Warrant  and the rights  evidenced
thereby, and does irrevocably constitute and appoint __________________________,
attorney, to transfer that part of said Warrant on the books of Hybridon, Inc.

Dated:_______________                     Signature:___________________________

Address:_____________________________

7

SUBORDINATION AND INTERCREDITOR AGREEMENT

THIS  SUBORDINATION AND INTERCREDITOR  AGREEMENT (this  "Agreement") is
effective as of December 7, 1999 (the  "Effective  Date") by and among Hybridon,
Inc., a Delaware corporation  ("Borrower"),  those persons who from time to time
hold the 8% senior  notes  (described  herein) of the  Borrower due November 30,
2002  (collectively,  the "Senior  Lenders"),  and Forum  Capital  Markets,  LLC
("Forum"),  Delaware State Employees  Retirement Fund,  Declaration of Trust for
the Defined  Benefit Plans of ICI American  Holdings Inc.,  Declaration of Trust
for the Defined Benefit Plans of Zeneca Holdings Inc., The J.W. McConnell Family
Foundation  and General  Motors  Employees  Domestic  Group Trust (said  trusts,
foundation and fund being referred to collectively as the "Pecks Parties"; Forum
and  the  Pecks  Parties  are  collectively  referred  to  as  the  "Subordinate
Lenders").

RECITALS:

A. Senior  Lenders have agreed to extend  financial  accommodations  to
Borrower pursuant to the terms of the Senior Loan Documents (defined below).

B.  Subordinate  Lenders are also  shareholders  of  Borrower  and have
representatives on Borrower's Board of Directors.

C. As of December 31, 1996,  Borrower entered into a non-revolving term
loan with Silicon Valley Bank (the "Bank") which was evidenced, in part, by that
certain  Loan  and  Security  Agreement  dated  as of  December  31,  1996  (the
"Subordinate Loan Agreement")

D. As security for the  financial  accommodations  made pursuant to the
Subordinate  Loan  Agreement,  Borrower  granted to Bank a security  interest in
certain  assets  of  Borrower  described  more  fully  in the  Subordinate  Loan
Agreement and herein.

E. On or about  November 20, 1998,  Subordinate  Lenders  purchased the
interests of Bank in credit  facility  evidenced and secured by the  Subordinate
Loan Agreement and the Subordinate Loan Documents (defined herein).

F. As a condition to making their new financial accommodations.  Senior
Lenders have  required,  and the Borrower and  Subordinate  Lenders have agreed,
that certain obligations of Borrower to Subordinate Lenders be subordinated, and
other processes be agreed to, as more fully set forth herein.

NOW,  THEREFORE,  in  consideration  of the foregoing  Recitals and the
mutual  covenants  and  agreements  set forth  herein,  and for  other  good and
valuable consideration,  the mutuality,  receipt and sufficiency of which hereby
are acknowledged,  and intending to be legally bound the parties hereto agree as
follows:

1.       DEFINITIONS.

Certain terms used herein and not otherwise defined (including capitalized terms
used in the foregoing Recitals) shall have the following meanings:

1

An "Acceleration"  shall mean the occurrence of any acceleration of the
principal and interest under any of the Borrower Obligations.

"Borrower's  Public  Filings" shall mean the periodic  filings on Forms
10-K,  10-Q and 8-K,  as filed  from time to time with the U.S.  Securities  and
Exchange Commission.

Borrower  Obligations"  means the Senior Obligations or the Subordinate
Obligations, as the context requires.

"Committee Event" shall have the meaning set forth in Section 2.2(a).

"Default"  shall mean any Default or "default"  under and as defined in
the Senior Loan  Agreement or the  Subordinate  Loan  Documents,  as the context
requires.

"Event of  Default"  means any Senior  Event of Default or  Subordinate
Event of Default, as the context requires.

"Forum Representative" means Harold L. Purkey, or a successor chosen by
Forum.

"Lenders Committee" shall have the meaning set forth in Section 2.2(a).

"Payment in Full" or "Paid in Full" or any similar  term) with  respect
to any Borrower  Obligation  means (a) the  indefeasible  satisfaction and final
payment  in full  of  such  Borrower  Obligation  in  cash  or cash  equivalents
reasonably  acceptable to the payee and the termination of any obligation on the
part of the holder of such  Borrower  Obligation  to make any loans or to afford
any financial  accommodation to Borrower and the full and timely  performance of
all other  obligations  to the holder of such Borrower  Obligation or (b) in the
case of any Borrower Obligation consisting of contingent  obligations (including
without  limitation  contingent  obligations  in respect of letters of credit or
other indemnifications under the Subordinate Loan Documents),  the setting apart
of cash  sufficient to discharge such portion of such Borrower  Obligation in an
account for the exclusive benefit of the holders thereof,  in which account such
holders  shall be  granted  by  Borrower  a first  priority  perfected  security
interest in a manner  acceptable  to such  holders,  which  payment or perfected
security  interest shall have been retained by the holders,  in the case of each
of (a)  and  (b)  above,  for a  period  of time  in  excess  of all  applicable
preference or other similar periods under applicable  bankruptcy,  insolvency or
creditors' rights laws.

"Pecks  Representative"  means Arthur W. Berry or a successor chosen by
the holders of a majority of the interests held by the Pecks Parties.

"Remedy  Notification"  means the written  notification  by Subordinate
Lenders to Senior Lenders or by Senior  Lenders to  Subordinate  Lenders of such
party's  desire to exercise a Remedy  following  the  occurrence  of an Event of
Default.

2

"Remedy"  means any the following  actions by either Senior  Lenders or
Subordinate Lenders:

(i)    the  exercise  of any  right or  remedies  they may have  under  the
Subordinate  Loan  Documents or otherwise  (other than a  declaration  of an
Acceleration);

(ii)   the  commencement or joinder with any other creditors of Borrower in
commencing  any  bankruptcy,  reorganization,   receivership  or  insolvency
proceeding against Borrower; or

(iii)  the  commencement  of any action or proceeding  against  Borrower to
enforce or collect any Borrower Obligation, to obtain possession of property
of Borrower,  to exercise  control  over  property of Borrower or to create,
perfect or enforce any lien against property of Borrower.

"Senior  Event of  Default"  means  any Event of  Default  under and as
defined in the Senior Loan Documents.

"Senior   Lenders'    Representative"    means   Youssef   El-Zein   (a
representative   designated   by  Pillar   Investments   Ltd.)  or  a  successor
representative  chosen by the holders of a majority  (measured by dollar amount)
of the Senior Obligations, outstanding from time to time.

"Senior Loan Documents" means the Borrower's 8% notes, due November 30,
2002, issued to Senior Lenders, the Subscription Agreements between Borrower and
each Senior  Lender,  the Warrant  Agreements  between  Borrower  and the Senior
Lenders  and all other  instruments,  agreements  and  documents  which  create,
evidence or secure the Senior  Obligations  from time to time (including but not
limited  to  any  promissory  notes,  security  agreements,  pledge  agreements,
hypothecation  agreements,   mortgages,  financing  statements,  and  all  other
agreements of any type whatsoever),  delivered by Borrower to Senior Lenders, as
such may be amended, modified,  supplemented,  restated,  replaced or refinanced
(in any such case with any Senior Lender) from time to time,  including all such
extensions,  renewals,  refinancings or refundings  thereof,  whether or not the
principal amount is increased.

"Senior  Obligations"  means all  obligations of the Borrower under the
Senior Loan Documents including but not limited to principal, interest, fees and
all other amounts owing to Senior Lenders under the Senior Loan Documents,  from
time to time.  Notwithstanding  the foregoing,  the Senior Obligations shall not
include any  principal  owed by the Borrower to the Senior  Lenders in excess of
$10,000,000  except with the consent of the Senior Lenders'  Representative  and
the Subordinate Lenders' Representatives.

"Subordinate  Debt"  means  all  principal,  interest,  fees and  other
amounts owing to Subordinate  Lenders under the Subordinate  Loan Documents from
time to time, whether in respect of principal interest or otherwise.

"Subordinate  Event of Default" means any Event of Default under and as
defined in the Subordinate Loan Documents.

"Subordinate   Lenders'   Representatives"   shall   mean   the   Pecks
Representative and the Forum Representative.

"Subordinate  Loan  Agreement"  shall have the meaning set forth in the
Recitals.

3

"Subordinate  Loan Documents" means  Subordinate Loan Agreement and all
other instruments, agreements and documents which create, evidence or secure the
Subordinate Obligations from time to time.

"Subordinate  Obligations"  means all obligations of Borrower under the
Subordinate  Loan  Documents  including but not limited to principal,  interest,
fees and all other amounts owing to  Subordinate  Lenders under the  Subordinate
Loan Documents, from time to time.

2.       SUBORDINATION AND INTERCREDITOR PROVISIONS.

2.1 Subordination.

(a) Subordinate  Lenders hereby consent to Borrower  obtaining  certain
financial accommodations from Senior Lenders, all on a senior secured basis.

(b) Senior Lenders hereby  acknowledge  that  Subordinate  Lenders have
been  previously  granted  a  security  interest  in  certain  of the  assets of
Borrower. Subordinate Lenders hereby acknowledge and agree that they are willing
to and hereby do  subordinate  the  Subordinate  Obligations  and the collateral
securing such obligations to the Senior Obligations.

(c)  Borrower  and  Subordinate  Lenders  each  hereby  represents  and
warrants  to Senior  Lenders  that a true,  accurate  and  complete  copy of all
Subordinate  Loan  Documents  has been either filed as an inhibit to  Borrower's
Public Filings or otherwise  provided to Senior Lenders'  Representative  or its
counsel in writing,  and that none of the  Subordinate  Loan  Documents has been
amended or modified in any way from the versions so filed or provided.

(d) Subordinate Lenders agree, for themselves and each future holder of
the  Subordinate  Obligations,  that:  (i)  subject  to the  terms  hereof,  the
Subordinate  Debt is and shall be expressly  subordinate  and junior in right of
payment to all Senior Obligations until the Senior Obligations have been Paid in
Full; (ii) Subordinate  Lenders shall not accept additional  security or further
collateral to support the payment or performance of the Subordinate Debt, unless
the Senior  Lender is granted a lien or  security  interest  in such  additional
collateral,  and such lien or security  interest  in favor of Senior  Lenders is
senior to the lien of the  Subordinate  Lenders;  and (iii) Senior  Lenders have
advanced funds in reliance upon the  subordination  of the Subordinate  Debt and
the collateral securing such debt to the Senior Obligations.

2.2 Lenders Committee.

(a) Senior Lenders and Subordinate Lenders hereby agree to constitute a
"Lenders  Committee"  immediately upon the first to occur of the following:  (i)
the  occurrence  of  an  Acceleration,  or  (ii)  the  occurrence  of  a  Remedy
Notification (a "Committee Event").

(b) The  Lenders  Committee  shall have three  members  which  shall be
comprised of the Senior Lenders' Representative and the two Subordinate Lenders'
Representatives.  Any  matter  which,  under  the  terms  of this  Agreement  or
otherwise, requires a vote or action by the Lenders Committee, shall require the
affirmative votes of a majority of the members of the Lenders Committee.

4

(c) From and after its  formation  following  a  Committee  Event,  the
Lenders  Committee shall be charged solely with  liquidating any collateral held
by any of the Senior Lenders or Subordinate  Lenders by obtaining  possession of
or exerting control over such  collateral,  and perfecting or enforcing liens of
the Senior Lenders and the Subordinate Lenders against such collateral. Borrower
and the Lenders  Committee  shall  disburse  any  proceeds  of such  liquidation
according to the priorities set by this Agreement.

(d) From and after any Event of  Default,  neither  Senior  Lenders nor
Subordinate  Lenders may exercise a Remedy without first providing not less that
ten (10) days advance  written  notice to the other  Lenders of its desire to so
exercise a Remedy (the "Remedy Notification"). Subsequent to the delivery of the
Remedy Notification and the resulting formation of the Lenders Committee,  then,
until the date the  Senior  Obligations  are Paid in Full,  Subordinate  Lenders
shall not exercise any Remedy  without  either (a)  direction or approval by the
Lenders  Committee or (b) express approval  provided herein.  Similarly,  at any
time  prior to the date the  Subordinate  Obligations  are Paid in Full,  Senior
Lenders shall not exercise any Remedy,  without either (a) direction or approval
by the Lenders Committee or (b) express approval provided herein.

(e)  If  any   insolvency,   bankruptcy,   receivership,   liquidation,
reorganization or other similar proceedings are commenced by or against Borrower
or its property, if any proceedings for involuntary liquidation,  dissolution or
other winding up of Borrower  whether or not involving  insolvency or bankruptcy
are  commenced  by  or  against  Borrower  (collectively,   any  "Reorganization
Proceedings"),  then Senior Lenders shall be entitled in any such Reorganization
Proceedings  to  receive  Payment  in  Full  of all  Senior  Obligations  before
Subordinate  Lenders  are  entitled  in any the  Reorganization  Proceedings  to
receive  any  payment  on  account  of  the  Subordinate  Obligations.   In  any
Reorganization  Proceedings,   any  payment  or  distribution  of  any  kind  or
character,  whether in cash or in property to which Subordinate Lenders would be
entitled on account of the  Subordinate  Obligations  but for the  provisions of
this Agreement,  shall be delivered to Senior Lender to the extent  necessary to
make Payment in Full of all Senior  Obligations  remaining unpaid,  after giving
effect to any  concurrent  payment or  distribution  to or for Senior  Lender in
respect thereof.  Subject to the Payment-in-Full of all Senior Obligations,  the
holders of  Subordinate  Obligations  shall be  subrogated  to the rights of the
holders of the Senior  Obligations  (to the extent of payments or  distributions
made to holders of Senior  Obligations  pursuant  to the  foregoing  sentence or
Section 2.3(b)) to receive  payments or  distributions of the assets of Borrower
applicable  to  the  Senior  Obligations.  No  such  payments  or  distributions
applicable  to the  Senior  Obligations  shall,  as  between  Borrower  and  its
creditors,  other than the  holders of Borrower  Obligations,  be deemed to be a
payment by Borrower to or on account of the Subordinate Obligations; and for the
purposes of such  subrogation,  no payments or  distributions  to the holders of
Senior  Obligations  to which the holders of  Subordinate  Obligations  would be
entitled  except for the provisions of this section shall,  as between  Borrower
and its creditors, other than the holders of Borrower Obligations,  be deemed to
be a payment by Borrower to or on account of the Senior Obligations.

(f)   Notwithstanding   anything  to  the  contrary  contained  herein,
Subordinate Lenders may, in any proceedings described in Section 2.2 (e), in the
name of Subordinate Lenders, file claims, proofs of claims and other instruments
of similar character necessary to enforce the obligations of Borrower in respect
of  the  Subordinate  Obligations.  Notwithstanding  anything  to

5

the contrary contained herein,  Senior Lenders may, in any proceedings described
in Section 2.2 (e), in the name of Senior Lenders, file claims, proofs of claims
and other instruments of similar character  necessary to enforce the obligations
of Borrower in respect of the Senior  Obligations.  Neither this Section  2.2(f)
nor any other provision  hereof shall be construed to give  Subordinate  Lenders
any right to vote any Borrower  Obligation held by Senior  Lenders,  any related
claim or any portion of such claim,  whether in connection  with any resolution,
arrangement,  plan  or  reorganization,   compromise,  settlement,  election  of
trustees or  otherwise,  all such  votes,  as to Senior  Obligations  to be made
solely on the direction of the Senior  Lenders.  Neither this Section 2.2(f) nor
any other  provision  hereof shall be construed to give Senior Lenders any right
to vote any Borrower Obligation held by Subordinate  Lenders,  any related claim
or any  portion  of such  claim,  whether  in  connection  with any  resolution,
arrangement,  plan  or  reorganization,   compromise,  settlement,  election  of
trustees or otherwise,  all such votes, as to Subordinate Obligations to be made
solely on the direction of the Subordinate Lenders.

2.3 Payments of Borrower Obligations.

(a) The following provisions shall govern Subordinate Lenders' right to
receive  and  Borrower's  right and  obligation  to pay any amount due and owing
under the Subordinate Loan Documents:

(i)    Provided that the  Subordinate  Lenders'  Representatives  shall not
have  been  notified  that  an  Acceleration  shall  have  occurred  and  be
continuing  or would be created  thereby  under the terms of the Senior Loan
Documents,  Subordinate  Lenders may receive and  Borrower  may pay interest
only at the interest rate set forth in the Subordinate  Loan Documents as of
the Effective Date, when due and owing on an unaccelerated  basis and not at
a rate applicable upon default.

(ii)   Except  as  expressly   permitted  pursuant  to  Section  2.3(a)(i),
Subordinate Lenders shall not be entitled to receive or retain any direct or
indirect  payment  (in  cash,  cash-equivalents,  property,  by  set-off  or
otherwise) of or on account of any Subordinate  Obligation at any time prior
to Payment in Full of the Senior Obligations;  provided,  however,  Borrower
may deliver to Subordinate Lenders' Representatives,  at any time (including
during the occurrence of an Event of Default under the Senior Loan Documents
and/or  the  Subordinate  Loan  Documents),  the  proceeds  from the sale of
Subordinate  Lender's  Collateral,  which  sale  shall  be made in a  manner
directed or approved by the Lenders Committee. Except as expressly permitted
pursuant to Section  2.3(a)(i)  and (ii), at any time that any of the Senior
Obligations is outstanding,  Borrower shall not make and Subordinate Lenders
shall not  receive  or  accept  any  payment  (in  cash,  cash  equivalents,
property,  by set-off,  "bid in" of debt in a  disposition  of collateral or
otherwise)   of  any  kind  or  nature  with  respect  to  the   Subordinate
Obligations.

(b) If  Subordinate  Lenders  receive any payment  with  respect to the
Subordinate  Obligations which Subordinate  Lenders are not permitted to receive
and retain pursuant to this Agreement,  then such payment shall be held in trust
for the  benefit  of, and shall be paid over  promptly  to Senior  Lenders,  for
application to the payment of the Senior Obligations,  in such order of priority
as Senior Lenders'  Representative  shall determine.  If Subordinate Lenders pay
over any  payment  or  distribution  as  provided  above,  then such  payment or
distribution  shall be

6

deemed to have been  made by  Borrower  directly  to Senior  Lenders  and not to
Subordinate Lenders and no Subordinate  Obligation shall be discharged by reason
of its  receipt of any payment or  distribution  which is so paid over to Senior
Lenders.

(c) To the extent  necessary for Senior Lenders to realize the benefits
of the  subordination  of  the  Subordinate  Obligations  provided  for  herein,
Subordinate Lenders shall execute and deliver to Senior Lenders'  Representative
such instruments or documents (together with such assignments or endorsements as
Senior Lender shall deem  necessary),  as are consistent  with the terms of this
Agreement and are reasonably requested by Senior Lenders' Representative.

(d) In the event  Subordinate  Lenders at any time incur any obligation
to pay money to Borrower, Subordinate Lenders hereby irrevocably agree that they
shall pay such  obligation in cash or cash  equivalents  in accordance  with the
terms of the document or instrument  governing such obligation without deduction
or set-off against the Subordinate Obligations.

2.4 Borrower's  Obligations Absolute.  The provisions of this Agreement
are solely for the benefit of Borrower,  Senior Lenders and Subordinate  Lenders
for the purpose of defining the relative rights of the parties thereto.  Nothing
herein  shall  impair,  as between  Borrower  and any other  party  hereto,  the
obligations of Borrower, which are unconditional and absolute, to Senior Lenders
and to Subordinate Lenders, respectively.

2.5 Transfers.  Any Senior Lender or any  Subordinate  Lender may sell,
assign  or  otherwise  transfer,  in  whole  or in  part,  any of  the  Borrower
Obligations or any interest  therein to any other person or entity,  but only on
the express  condition  that the  transferee of the Borrower  Obligations  shall
expressly  acknowledge to the other parties to this agreement,  in writing, that
it agrees to be bound by all of the terms hereof. Senior Lenders and Subordinate
Lenders  each  hereby  represents  and  warrants  to the  others  that as of the
execution  date  hereof  neither  Senior  Lenders  nor  Subordinate  Lenders has
transferred  or entered  into any  agreement  or  understanding  with a proposed
transferee that they will transfer any of the Borrower Obligations.

2.6 Liens  Subordinate.  (a)  Subordinate  Lenders agree that any liens
upon  Borrower's  assets  securing  payment  of the  Subordinated  Debt,  now or
hereafter existing,  are and shall be and remain inferior and subordinate to any
liens  securing  payment of the Senior  Obligations  regardless  of whether such
encumbrances in favor of the  Subordinated  Lenders or Senior Lenders  presently
exist or are hereafter created or attach.

(b) Senior Lenders and Subordinate Lenders hereby agree that, after the
Lenders Committee is constituted, the Lenders Committee may file any or all lien
releases,  UCC  releases,  and  termination  statements  on behalf of the Senior
Lenders and the  Subordinate  Lenders at any time  Borrower,  or any  successor,
assign or agent of  Borrower,  proposes a sale of any asset that is  approved by
the  Lenders  Committee.  In  furtherance  thereof,  the Senior  Lenders and the
Subordinate Lenders agree to execute, acknowledge and deliver any lien releases,
UCC-3 termination  statements or such additional instruments or documents as may
be reasonably  necessary to confirm the foregoing within three (3) business days
of the request therefor by Lenders Committee.

7

2.7 Additional Representations and Warranties.  Subordinate Lenders and
Borrower represent and warrant to Senior Lenders that:

(a)  as  of  the  date  hereof,  the  total  principal  amount  of  the
Subordinate Obligations is $6,000,000.00 plus accrued but unpaid interest;

(b) except as indicated in  Borrower's  Public  Filings or disclosed in
writing to the Senior Lenders'  Representative and its counsel, which writing is
hereby  made a part  hereof,  as of the  date  hereof,  to  the  best  of  their
knowledge, after due inquiry, no default or Event of Default, or event which the
notice or passage of time or both would constitute an Event of Default exists or
has occurred under the Subordinate Loan Documents;

(c)  Subordinate  Lenders  are  collectively  the  exclusive  legal and
beneficial owner of all of the Subordinate Obligations;

(d) except as indicated in  Borrower's  Public  Filings or disclosed in
writing to the Senior Lenders'  Representative and its counsel, which writing is
hereby made a part of this  Agreement,  none of the  Subordinate  Obligations is
subject  to  any  lien,  security  interest  (other  than  Subordinate  Lender's
Collateral), financing statements, subordination, assignment or other claim; and

(e) this Agreement constitutes the legal, valid and binding obligations
of Subordinate: Lenders, enforceable in accordance with its terms.

2.8 Legends.  Subordinate Lenders agree that any instrument at any time
evidencing  the  Subordinate  Obligations,  or any  portion  thereof,  shall  be
permanently  marked  on its face  with a legend  conspicuously  indicating  that
payment thereof is subordinate in right of payment to the Senior Obligations and
subject to the terms and conditions of this Agreement, and after being so marked
certified copies thereof shall be delivered to Senior Lenders.  In the event any
legend  or   endorsement   is   omitted,   Senior   Lenders   or  any  of  their
representatives,  officers or employees  are hereby  irrevocably  authorized  on
behalf of  Subordinate  Lenders to make the same.  No specific  legend,  further
assignment or endorsement or delivery of notes,  guarantees or instruments shall
be necessary to subject any Subordinate Obligations to the subordination thereof
contained in this Agreement.

2.9 Waiver of Covenant.  Subordinate  Lenders hereby waive any breaches
or defaults arising from Borrower's failure to maintain  compliance with Section
6.9 of the Subordinate Loan Agreement, entitled "Minimum Liquidity", such waiver
to  remain  in  effect  so long as any  amounts  of  Senior  Obligations  remain
outstanding,

3. AGREEMENT BY BORROWER.

(a) Borrower hereby  acknowledges and agrees to the foregoing terms and
provisions,  and agrees that the provisions hereof will bind Borrower,  together
with its successors and assigns.

(b) Borrower acknowledges and agrees that: (i) in the event of a breach
by Borrower or Subordinate Lenders of any of the terms and provisions  contained
in this  Agreement,  such a

8

breach shall constitute an Event of Default,  as defined in and under the Senior
Loan Documents;  and (ii) it will execute and deliver such additional  documents
and take such additional  action as may be necessary or desirable in the opinion
of either Subordinate Lenders or Senior Lenders to effectuate the provisions and
purposes of this Agreement.

4. MISCELLANEOUS.

4.1  Notices.  Any  and all  notices  given  in  connection  with  this
Agreement shall be deemed  adequately  given only if in writing and addressed to
the party for whom such notices are intended at the address set forth below. All
notices shall be sent by personal delivery,  Federal Express or other over-night
messenger  service,  first class registered or certified mail,  postage prepaid,
return  receipt  requested  or by other  means at least as fast and  reliable as
first class  mail.  A written  notice  shall be deemed to have been given to the
recipient  party on the  earlier  of (a) the date it shall be  delivered  to the
address  required  by this  Agreement;  (b) the date  delivery  shall  have been
refused  at the  address  required  by this  Agreement;  or (c) with  respect to
notices  sent by mail,  the date as of  which  the  postal  service  shall  have
indicated  such  notice to be  undeliverable  at the  address  required  by this
Agreement.  Any and all notices  referred to in this Agreement,  or which either
party desires to give to the other, shall be addressed as follows:

if to Borrower:                     Hybridon. Inc.
155 Fortune Blvd.
Milford, MA 01757
Attn.: President

with a copy to:                     Kramer Levin Naftalis & Frankel LLP
919 Third Avenue
New York, New York 10022
Attn.: Monica C. Lord. Esq.

if to Senior Lenders:               Pillar Investments Ltd. Representative
28 Avenue de Messine
Paris, FRANCE 75008
Attn: Youssef El-Zein

with a copy to:                     Sachnoff & Weaver, Ltd.
30 South Wacker Drive
Suite 2900
Chicago, Illinois 60606
Attn: Lance R. Rodgers, Esq.

if to Subordinate Lenders:          Pecks Management
1 Rockefeller Plaza,
Suite 900
New York, NY 10020
Attn: Arthur W. Berry
and
Forum Capital Markets

9

53 Forest Avenue
Old Greenwich, CT 06870
Ann: Harold L. Purkey

with copies to:

The above addresses may be changed by notice of such change,  mailed as provided
herein, to the last address designated.

4.2 No Fiduciary Duty.  Nothing in this Agreement shall be construed to
create or impose upon any Senior  Lender any fiduciary  duty to any  Subordinate
Lender,  or any other  implied  obligation  to act or refrain  from  acting with
respect  to  Borrower  or the  Senior  Obligations  or the  collateral  security
securing the Senior Obligations in any manner contrary to what any Senior Lender
may determine is in its own best interests. Similarly, nothing in this Agreement
shall be construed to create or impose upon any Subordinate Lender any fiduciary
duty to any Senior  Lender,  or any other  implied  obligation to act or refrain
from acting  with  respect to Borrower  or the  Subordinate  Obligations  or the
collateral security securing the Subordinate  Obligations in any manner contrary
to what any Subordinate Lender may determine is in its own best interests.

4.3 Notice of Default.  In addition to any other  notices  which may be
required  hereunder,  Subordinate  Lenders  shall give written  notice to Senior
Lender  Representative,  promptly  after they become aware of the occurrence of:
(a) an Event of Default under the terms of the Subordinate  Loan Documents;  (b)
the  cure  of any  such  Event  of  Default:  (c)  the  payment  in  full of the
Subordinate  Debt; (d) any  Acceleration  of the  Subordinate  Debt; and (e) any
action or  proceeding  instituted  against  Borrower  on account of any Event of
Default.

4.4 Successors; Continuing Effect.

(a) This  Agreement is being entered into for the benefit of, and shall
be binding upon,  Borrower,  each Senior Lender and each Subordinate  Lender and
their respective successors and assigns, including each subsequent or additional
holder of Senior  Obligations or Subordinate Debt, and any participant  (whether
now existing or hereafter arising) in the Senior Obligations.  The terms "Senior
Lenders"  and  "Subordinate  Lenders"  shall  include,  respectively,  any  such
subsequent or  additional  holder of or  participant  in Senior  Obligations  or
Subordinate Obligations whenever the context permits. This Agreement shall inure
to the  benefit of and be  enforceable  by any  future  holder or holders of the
Borrower  Obligations  or any part of any of the same;  provided  that,  nothing
contained  in this  Section  4.3 shall be deemed to permit the  transfer  of the
Subordinate Obligations in violation of the provisions of Section 2.5.

(b) Senior  Lenders  reserve the right to grant  participations  in, or
otherwise  sell,  assign,  transfer  or  negotiate  all or any part  of,  or any
interest  in,  the Senior  Obligations  and the  Collateral  securing  same.  In
connection  with any  participation  or other  transfer  or  assignment,  Senior
Lenders (i) may disclose to such assignee,  participant  or other  transferee or
assignee all documents and information  which Senior Lender now or hereafter may
have  relating  to the  Senior  Obligations  or the  Collateral,  and (ii) shall
disclose to such  participant or other  transferee or assignee the existence and
terms and conditions of this Agreement.

10

4.5  Amendments.  This  Agreement  may be  amended  only  by a  written
instrument executed by holders of a majorities in interest of each of the Senior
Obligations  and the  Subordinate  Obligations  and, if such  amendment  affects
Borrower, by Borrower.

4.6 Term.  This  Agreement  shall remain in full force and effect until
the Payment in Full of the Senior Obligations.

4.7  Waivers.  No waiver shall be deemed to be made by any party of any
of its rights  hereunder  unless the same shall be in writing and then only with
respect to the specific  instance  involved,  and no such waiver shall impair or
offset the rights of the waiving party or the obligations of the party benefited
by such waiver in any other respect or at any other time.

4.8 Governing Law. This  Agreement,  including the validity  hereof and
the rights and  obligations of the parties  hereunder,  shall be governed by and
construed  and  enforced  in  accordance  with the laws of the  Commonwealth  of
Massachusetts.

4.9 The  Borrower  May Not  Impair  Subordination.  No right of  Senior
Lenders or Subordinate Lenders to enforce the subordination created hereby shall
be  impaired  by any act or  failure  to act by  Borrower  or by the  failure by
Borrower to comply with this  Agreement,  regardless of any knowledge  which any
Senior Lender or any Subordinate Lender may have or be otherwise charged with.

4.10 Specific  Performance.  The parties hereto  acknowledge that legal
remedies maybe inadequate and therefore  Senior Lenders and Subordinate  Lenders
are hereby  authorized to demand specific  performance of the provisions of this
Agreement at any time when Borrower. Senior Lenders or Subordinate Lenders shall
have  failed to comply  with any  provision  hereof.  Each party  hereto  hereby
irrevocably  waives any  defense  based on the  adequacy of a remedy at law that
might be asserted as a bar to such remedy of specific performance.

4.11 Further Actions.  After the execution of this Agreement each party
will  execute and deliver all such  documents  and  instruments  and do all such
other acts and things as may be reasonably necessary to carry out the provisions
of this Agreement.

4.12  Agreement  to  Control.  If  any  provision  in any  document  or
instrument  relating to the Senior  Obligations or the Subordinate  Debt differs
with the terms of this Agreement  regarding the same or any similar matter,  the
provisions of this  Agreement  shall control and each other  provision  shall be
interpreted so as to give effect to the provisions of this Agreement.

4.13 Entire Agreement.  This Agreement contains the entire agreement of
the parties with respect to the subject  matter hereof and  supersedes all prior
written and oral agreements,  and all contemporaneous oral agreements,  relating
to such matters.

4.14 Severability.  Any provision of this Agreement which is prohibited
or  unenforceable  in  any  jurisdiction  shall,  as  to  such  jurisdiction  be
ineffective  to the  extent  of such  prohibition  or  unenforceability  without
invalidating  the  remaining  portions  hereof  or  affecting  the  validity  or
enforceability of such provision in any other jurisdiction.

11

4.15  Facsimile.  For  purposes  of  negotiating  and  finalizing  this
Agreement  (including any subsequent  amendments  thereto),  any signed document
transmitted  by  facsimile  machine  ("Fax")  shall be treated in all manner and
respects as an original  document.  The  signature  of any party by Fax shall be
considered  for these purposes as an original  signature.  Any such Fax document
shall be  considered  to have the  same  binding  legal  effect  as an  original
document,  provided  that an original of the faxed  document was mailed by first
class U.S. Mail or  personally  delivered to the  recipient,  on the date of its
transmission  with proof of the fax  transmission.  At the request of any party,
any Fax document  subject to this Agreement shall be re-executed by both parties
in an original  form.  The  undersigned  parties hereby agree that neither shall
raise  the use of the  Fax or the  fact  that  any  signature  or  document  was
transmitted  or  communicated  through  the  use  of a Fax as a  defense  to the
formation  of  this  Agreement.  This  agreement  may be  signed  in one or more
counterparts,  each of which  shall be an  original,  but all of which  together
shall   constitute  one  agreement.   binding  on  all  of  the  parties  hereto
notwithstanding  that all of the parties hereto are not  signatories to the same
counterpart.  Each of the undersigned  parties authorizes the assembly of one or
more original  copies of this Agreement  through the  combination of the several
executed counterpart  signature pages with one or more copies of this Agreement.
including  the  Schedules  and  Exhibits,  if any to this  Agreement.  Each such
compilation of this Agreement shall constitute one original of this Agreement.

4.16 Consent to Jurisdiction; Waiver of Jury Trial.

(a) BORROWER,  SUBORDINATE LENDERS AND SENIOR LENDER EACH HEREBY (i) TO
THE EXTENT PERMITTED BY APPLICABLE LAW,  IRREVOCABLY SUBMITS TO THE JURISDICTION
OF ANY STATE OR FEDERAL COURT LOCATED IN BOSTON, MASSACHUSETTS,  OVER ANY ACTION
OR  PROCEEDING  TO ENFORCE OR DEFEND ANY MATTER  ARISING FROM OR RELATED TO THIS
AGREEMENT;  (ii) IRREVOCABLY WANES, TO THE FULLEST EXTENT BORROWER,  SUBORDINATE
LENDERS AND SENIOR LENDERS MAY EFFECTIVELY DO SO, THE DEFENSE OF AN INCONVENIENT
FORUM TO THE  MAINTENANCE  OF ANY SUCH ACTION OR  PROCEEDING  IN ANY SUCH COURT;
(iii) AGREES THAT, TO THE EXTENT  PERMITTED BY APPLICABLE  LAW, A FINAL JUDGMENT
IN ANY SUCH ACTION OR PROCEEDING  IN ANY SUCH COURT SHALL BE CONCLUSIVE  AND MAY
BE ENFORCED IN ANY OTHER  JURISDICTION  BY SUIT ON THE  JUDGMENT OR IN ANY OTHER
MANNER  PROVIDED BY LAW;  AND (iv) TO THE EXTENT  PERMITTED BY  APPLICABLE  LAW,
AGREES NOT TO INSTITUTE ANY LEGAL ACTION OR PROCEEDING  AGAINST ANY PARTY HERETO
OR ANY OF PARTY'S DIRECTORS, OFFICERS, EMPLOYEES, AGENTS OR PROPERTY, CONCERNING
ANY MATTER  ARISING OUT OF OR RELATING TO THIS AGREEMENT 1N ANY COURT OTHER THAN
ONE LOCATED IN BOSTON, MASSACHUSETTS.

(b)  NOTHING  IN  THIS  SECTION   SHALL  AFFECT  OR  IMPAIR  SENIOR  OR
SUBORDINATE  LENDERS'  RIGHT TO SERVE  LEGAL  PROCESS ON  BORROWER IN ANY MANNER
PERMITTED BY LAW OR SENIOR OR SUBORDINATE  LENDERS' RIGHT TO BRING ANY ACTION OR
PROCEEDING  AGAINST  BORROWER OR BORROWER'S  PROPERTY IN THE COURTS OF ANY OTHER
JURISDICTION.

12

(c)  BORROWER,  SENIOR  LENDERS  AND  SUBORDINATE  LENDERS  EACH HEREBY
KNOWINGLY,  VOLUNTARILY AND INTENTIONALLY WAIVES THE RIGHT TO A TRIAL BY JURY IN
RESPECT OF ANY LITIGATION  BASED HEREON,  ARISING OUT OF, UNDER OR IN CONNECTION
WITH THIS AGREEMENT OR ANY OTHER LOAN DOCUMENT OR ANY COURSE OF CONDUCT,  COURSE
OF DEALINGS,  STATEMENTS (WHETHER VERBAL OR WRITTEN) OR ACTIONS OF EITHER PARTY.
EACH  PARTY  HERETO  HEREBY  EXPRESSLY  ACKNOWLEDGES  THIS  WAIVER IS A MATERIAL
INDUCEMENT  FOR SENIOR LENDER TO ENTER INTO THIS  AGREEMENT AND TO MAKE THE LOAN
EVIDENCED BY THE SENIOR LOAN DOCUMENTS.

IN WITNESS  WHEREOF,  the  parties  hereto  have  executed  and  delivered  this
Subordination Agreement as of the day, month and year first above written.

HYBRIDON, INC.

By:_______________________________
Name: ____________________________
Title:____________________________

SENIOR LENDERS
By: Pillar Investments Ltd, 
Their Representative

By:_______________________________
Name:_____________________________

FORUM CAPITAL MARKETS. LLC

By:_______________________________
Name:_____________________________
Title:____________________________

DELAWARE STATE EMPLOYEES RETIREMENT
FUND  DECLARATION  OF TRUST FOR THE
DEFINED   BENEFIT   PLANS   OF  ICI
AMERICAN  HOLDINGS INC  DECLARATION
OF TRUST  FOR THE  DEFINED  BENEFIT
PLANS OF ZENECA  HOLDINGS  INC. THE
J.W.  MCCONNELL  FAMILY  FOUNDATION
GENERAL MOTORS  EMPLOYEES  DOMESTIC
GROUP TRUST

By: PECKS MANAGEMENT PARTNERS, LTD.

By:_______________________________

[LETTERHEAD OF PILLAR INVESTMENTS]

December __, 1999

Hybridon, Inc.
155 Fortune Boulevard
Milford, MA  01757
Attn:  Mr. E. Andrews Grinstead, III.

Ladies and Gentlemen:

This   letter   agreement   (this   "Agreement")   is  to  confirm  our
understanding  regarding  the  compensation  to be paid by Hybridon,  Inc.  (the
"Company")  to  Pillar   Investments  Ltd.  and  its  affiliates  and  designees
(collectively  "Pillar") in  connection  with the  Company's  private  placement
offering (the "Offering") of Notes due 2002 ("Notes").

The Company shall issue to Pillar  additional Notes (the "In-Kind Fee")
in an aggregate  principal  amount  equal to nine percent (9%) of the  aggregate
principal  amount of Notes purchased in the Offering by investors  introduced to
the Company by Pillar.

The  Company  shall  issue  to  Pillar  warrants  (the  "Unit  Purchase
Warrants") to purchase  additional Notes in an aggregate  principal amount equal
to ten percent (10%) of the aggregate principal amount of Notes purchased in the
Offering by investors  introduced  to the Company by Pillar.  The Unit  Purchase
Warrants  shall be  exercisable  until and  including  November  30,  2006 at an
exercise price equal to 110 percent (110%) of the Notes' principal amount.

The Company  shall pay in cash all  reasonable  out-of-pocket  expenses
incurred by Pillar in providing services with respect to the Offering, including
reasonable fees and disbursements of Pillar's  counsel,  within thirty (30) days
of  submission  of  a  bill  or  bills   accompanied   by  reasonably   detailed
documentation by Pillar.

Notwithstanding the foregoing,  the Company's  obligations to issue the
In-Kind Fee and the Unit Purchase Warrants and to reimburse  Pillar's  Placement
Expenses pursuant to this Agreement shall be subject to the condition  precedent
that the Company will have had  delivered  to it a fairness  opinion in form and
substance  deemed  by the  Company,  in its  sole  discretion,  to  satisfy  the
requirements  of  that  certain  Indenture,   relating  to  the  9%  Convertible
Subordinated Notes Due 2004 of the Company, between the Company and State Street
Bank and Trust Company, dated as of March 26, 1997, as amended.

This  Agreement  shall be governed by and construed in accordance  with
the laws of the State of New York without  regard to  principles of conflicts of
law. The parties hereto

Hybridon, Inc.
December __, 1999
Page 2

irrevocably  consent to the  jurisdiction of the courts of the State of New York
and of any federal court located in such State in connection  with any action or
proceeding  arising  out of or  relating  to this  Agreement,  any  document  or
instrument delivered pursuant to, in connection with or simultaneously with this
Agreement, or a breach of this Agreement or any such document or instrument.  In
any such action or proceeding,  each party hereto waives personal service of any
summons,  complaint or other process and agrees that service thereof may be made
in accordance with this  Paragraph.  Within thirty (30) days after such service,
or such other time as may be mutually  agreed  upon in writing by the  attorneys
for the parties to such action or  proceeding,  the party so served shall appear
or answer such  summons,  complaint or other  process.  Pillar  hereby  appoints
Sachnoff & Weaver Ltd.  as its agent for  purposes  of notice  hereunder  and to
receive  on behalf of Pillar  service of copies of summons  and  complaints  and
other  process  which may be served in any such action or  proceeding.  All such
notices to Pillar  shall be sent to  Sachnoff & Weaver,  Ltd.,  30 South  Wacker
Drive, Suite 2900, Chicago, Illinois 60606, Attn: Lance R. Rodgers, Esq.

This  Agreement  shall be binding  upon  Pillar and the Company and the
successors and assigns of Pillar.

This Agreement is intended, and for all purposes shall be construed, to
supersede any existing agreements,  whether written, oral or otherwise,  between
the parties hereto regarding  Pillar's rights to compensation in connection with
the Offering.

Please   confirm  that  the  foregoing  is  in  accordance   with  your
understanding  by signing and  returning  to us the  enclosed  duplicate of this
Agreement.

Sincerely yours,
PILLAR INVESTMENTS LTD.

By:___________________________
Name:
Title:

Confirmed as of the date hereof:
HYBRIDON, INC.

By:__________________________________
Name:  E. Andrews Grinstead, III.
Title:  President and CEO

SUBSCRIPTION AGREEMENT

SUBSCRIPTION  AGREEMENT (this  "Agreement"),
dated as of December 13, 1999, by and among HYBRIDON,  INC., a
Delaware  corporation (the "Company"),  and the Persons listed
on the signature pages hereof (the "Current  Purchasers")  and
such other Persons that from time to time hereafter may become
party  hereto  pursuant  to  Section  13.10  (the  "Additional
Purchasers" and, collectively with the Current Purchasers, the
"Purchasers").

The  Company  desires  to issue  and sell to  Purchasers,  and
Purchasers desire to purchase from the Company,  Notes due 2002 (the "Notes") in
substantially  the form  attached  hereto as Exhibit A, upon and  subject to the
terms and conditions  hereinafter set forth. As used herein, the term "Offering"
shall mean the  offering  of Notes by the  Company  during the  Offering  Period
hereinafter  referred to pursuant to this  Agreement and  substantially  similar
agreements.

Accordingly,  in  consideration of the premises and the mutual
agreements contained herein, Purchasers and the Company hereby agree as follows:

1.  Purchase  and Sale of the Notes.  Subject to the terms and
conditions  set forth  herein,  the Company  hereby  agrees to issue and sell to
Purchasers, and Purchasers,  severally and not jointly, hereby agree to purchase
from the Company Notes.  The aggregate  purchase price for the respective  Notes
sold to each Purchaser  pursuant to this Agreement  shall be one hundred percent
(100%) of the aggregate principal amount of such Notes. "Operative Documents" as
used  herein  shall  mean this  Agreement,  the  Notes,  the  Subordination  and
Intercreditor  Agreement of even date herewith (the  "Intercreditor  Agreement")
and the form of Warrant  Agreement  attached  hereto as Exhibit B (the  "Warrant
Agreement").

2.       Delivery of Notes.

2.1.  Delivery of Notes. (a) The Company shall offer the Notes
for sale. Upon receipt of subscriptions for Notes pursuant to the Offering,  the
Company may conduct an initial closing (any closing  hereunder,  a "Closing" and
the date thereof,  a "Closing Date") and may conduct  subsequent  Closings on an
interim basis during the Offering Period. The Offering Period shall terminate at
12:00 noon (New York Time) on December  31,  1999,  subject to  extension at the
sole option of the Company, for an additional 60 days (the "Termination Date").

(b) Contemporaneously  with the execution and delivery of this
Agreement  by a  Purchaser  and  pending  the sale of Notes at a  Closing,  such
Purchaser  will be  required to deposit  the  Purchase  Price in escrow with the
Escrow  Agent (as defined in the Escrow  Agreement  hereinafter  referred to) by
wire transfer of immediately available funds for the account of the Escrow Agent
made payable to Sachnoff & Weaver, 30 South Wacker Drive,  29th Floor,  Chicago,
Illinois,  60606, Attention Douglas Newkirk,  pursuant to the terms of an escrow
agreement in  substantially  the form attached  hereto as Exhibit C (the "Escrow
Agreement").

(c) At a Closing,  the funds  required for the purchase of the
Notes by respective  Purchasers  will be released by the Escrow Agent net of any
Required Deductions (as

defined in the Escrow  Agreement) from the escrow account in accordance with the
terms of the Escrow  Agreement.  The Company will promptly deliver to Purchasers
the Notes to be  purchased  on the date of a Closing  as set forth in  Article 1
hereof  against  the  receipt by the  Company of the  Purchase  Price net of any
Required  Deductions from escrow in accordance with the Escrow  Agreement.  Each
Purchaser  hereby  authorizes  the Secured  Party (as  defined  below) to accept
delivery  of  Notes  on such  Purchaser's  behalf  unless  the  Purchaser  is in
attendance  at such Closing.  The Notes shall be  registered in the  Purchasers'
respective   names  or  the  name  of  the  nominee(s)  of  such  Purchasers  in
denominations of $1,000 and integral  multiples thereof pursuant to instructions
delivered to the Company not less than two days prior to a Closing.  Interest on
each Note sold in the  Offering  shall  accrue only from the date of issuance of
such Note.

2.2.  Warrants.  Pursuant to the Notes, if the Company prepays
the Notes (or any portions thereof), then the Company shall simultaneously issue
to the holders of such prepaid  Notes (or  portions)  warrants  ("Warrants")  to
purchase  a number of shares of Common  Stock  equal to the  number of shares of
Common Stock then issuable upon conversion of such prepaid Notes (or the prepaid
portions  thereof,  if prepaid in part).  The terms of the Warrants  shall be as
more fully described in the Warrant  Agreement  between the Company  ChaseMellon
Shareholder  Services  LLC, as Warrant Agent and the Secured Party (the "Warrant
Agreement"), a form of which is attached hereto as Exhibit B.

3.  Conditions to the  Obligations of Purchasers at a Closing.
The  obligation  of Purchasers to purchase and pay for the Notes to be purchased
by  Purchasers  at a Closing is subject to the  satisfaction  on or prior to the
relevant Closing Date of the following  conditions,  which may only be waived by
written consent of the Secured Party.

3.1.  Representations and Warranties.  The representations and
warranties of the Company  contained in this Agreement shall be true and correct
in all  material  respects  when  made,  and  shall be true and  correct  in all
material respects at and as of the date of such Closing as if they had been made
on and as of such Closing.

3.2.  Performance  of  Covenants.  All  of the  covenants  and
agreements  of the  Company  contained  in this  Agreement  and  required  to be
performed on or prior to the relevant  Closing Date shall have been performed in
a manner reasonably satisfactory to the Secured Party.

3.3.  Closing  Documents.  The Company shall have delivered to
the Secured Party the following:

(a) a certificate executed by the President or Chief Executive
Officer  of the  Company  dated  the  relevant  Closing  Date  stating  that the
conditions set forth in Sections 3.1 and 3.2 have been satisfied; and

(b) a certificate  of the Secretary or Assistant  Secretary of
the Company,  dated the relevant  Closing Date,  certifying the attached copy of
the By-laws of the Company,  the  authorization  of the execution,  delivery and
performance of the Operative  Documents,  and the  resolutions  authorizing  the
actions to be taken by the Company under the Operative Documents.

2

3.4. No Legal Order Pending. There shall not then be in effect
any legal or other order enjoining or restraining the transactions  contemplated
by this Agreement.

3.5. No Law Prohibiting or Restricting  Such Sale. There shall
not be in effect any law, rule or regulation  prohibiting  or  restricting  such
sale or  requiring  any consent or  approval of any person  which shall not have
been  obtained  to  issue  the  Notes  (except  as  otherwise  provided  in this
Agreement).

3.6. Proceedings. All corporate and other proceedings taken or
to be taken  in  connection  with the  transactions  contemplated  hereby  to be
consummated  at each  Closing  and  all  documents  incident  thereto  shall  be
reasonably  satisfactory  in form and substance to the Secured Party,  including
but not limited to the authorization of the issuance of the Notes,  Warrants and
the Note-Underlying Common Stock (as defined below).

4.  Conditions to the Obligations of the Company at a Closing.
The obligation of the Company to issue and sell Notes to Purchasers at a Closing
is subject to the satisfaction of the following conditions, each of which may be
waived by the Company:

4.1.  Representations and Warranties.  The representations and
warranties  of each  Purchaser  contained  in this  Agreement  shall be true and
correct  when made,  and shall be true and correct at and as of the date of such
Closing as if they had been made on and as of such Closing.

4.2. No Law Prohibiting or Restricting  Such Sale. There shall
not be in effect any law, rule or regulation  prohibiting  or  restricting  such
sale or  requiring  any consent or  approval of any person  which shall not have
been  obtained  to  issue  the  Notes  (except  as  otherwise  provided  in this
Agreement).

5.       Creation of Security Interest.

5.1. Grant of Security Interest. The Company hereby grants and
pledges to Youssef El-Zein (a  representative  designated by Pillar  Investments
Ltd.) or a successor  representative  chosen,  at any time,  by the holders of a
majority  (measured by dollar amount) of the Notes outstanding from time to time
(the "Secured Party"),  solely as agent for Purchasers and not in his individual
capacity, a continuing security interest in all presently existing and hereafter
acquired or arising  assets and  property of the Company  described on Exhibit D
hereto (the "Collateral") in order to secure prompt payment of the principal sum
and interest  evidenced by the Notes, and the performance by the Company of each
of its obligations  under this Agreement and the Notes.  Such security  interest
shall  automatically  terminate upon the earlier of (i) the payment of principal
and  interest  on the  Notes  and (ii)  such  time as the  Notes  are no  longer
outstanding  (the  "Security  Interest  Termination  Date").  Purchasers  hereby
acknowledge and agree that the security interests granted hereby are subordinate
and  subject to any prior  security  interest in the  Collateral  granted by the
Company, except as modified by the Intercreditor  Agreement, and are subordinate
and subject to any lien granted in the future to secure  Operating  Indebtedness
of the Company.  Purchasers  and the Secured  Party hereby agree not to exercise
any of their rights with respect to the Collateral under this Agreement, at law,
in equity or  otherwise  until the holders of Operating  Indebtedness  have been
paid in full.

3

5.2.  Designation  of Secured Party as Agent.  Purchasers,  by
their  acceptance  of the  benefits  of this  Agreement  and the  Notes,  hereby
irrevocably  designate the Secured Party to act as Secured Party with respect to
this Agreement and as specified in the other Operative Documents. Each Purchaser
hereby  irrevocably  authorizes,  and each holder of any Note,  by such holder's
acceptance of such Note, shall be deemed  irrevocably to authorize,  the Secured
Party to take such action on its behalf under the  provisions of this  Agreement
and the other  Operative  Documents  and any other  instruments  and  agreements
referred to herein or therein and to  exercise  such powers and to perform  such
duties  hereunder and thereunder as are  specifically  delegated to, or required
of, the Secured  Party by the terms  hereof or thereof and such other  powers as
are  reasonably  incidental  thereto.  Each  Purchaser,  on behalf of itself and
future  holders of the Notes issued to such  Purchaser,  hereby  authorizes  and
directs the Secured Party,  from time to time in the Secured Party's  discretion
to take any  action  and  promptly  to  execute  and  deliver  on its behalf any
document  or  instrument  that the  Company  may  reasonably  request to effect,
confirm  or  evidence  the  provisions  of this  Article 5,  including,  without
limitation,  the  occurrence  of the Security  Interest  Termination  Date,  any
subordination agreement, or otherwise. In addition, Purchasers and Secured Party
hereby  covenant and agree  promptly to execute and deliver any such document or
instrument in respect of such subordination, and in respect of the occurrence of
the Security Interest Termination Date, as the Company may reasonably request.

5.3.  Delivery  of  Additional   Documentation  Required.  The
Company  shall from time to time  execute and deliver to Secured  Party,  at the
request of Secured  Party,  all financing  statements  and other  documents that
Secured Party may reasonably  request to perfect and continue  perfected Secured
Party's  security  interests in the  Collateral and in order fully to consummate
all of the transactions  contemplated under this Agreement,  it being understood
and agreed by the  Purchasers  and the Secured  Party that the Company  need not
deliver  possession of any Collateral to the Secured  Party,  take any action to
perfect the security  interest granted hereby other than the filing of financing
statements  under the  Uniform  Commercial  Code or take any other  action  that
would,  in its sole  judgment,  conflict  with the terms of or pertaining to any
Operating Indebtedness.

6.  Representations  and Warranties of Purchasers.  Purchasers
hereby severally represent and warrant to the Company as follows:

6.1.  Investment  Intent.  Each Purchaser  recognizes that the
purchase of the Notes involves a high degree of risk including,  but not limited
to, the  following:  (i) the Company  remains a development  stage business with
limited  operating  history and  requires  substantial  funds in addition to the
proceeds  of  the  Offering;  (ii)  an  investment  in  the  Company  is  highly
speculative,  and  only  investors  who can  afford  the  loss of  their  entire
investment should consider investing in the Company, the Notes, the Warrants, or
the shares of Note-Underlying Common Stock, (iii) such Purchaser may not be able
to liquidate his investment; (iv) transferability of the Notes, the Warrants and
the  Note-Underlying  Common Stock is extremely  limited;  (v) in the event of a
disposition  of the Notes,  the Warrants and the  Note-Underlying  Common Stock,
such  Purchaser  could  sustain the loss of his entire  investment  and (vi) the
Company has not paid any dividends  since  inception and does not anticipate the
payment of dividends on the Common Stock in the foreseeable  future.  Such risks
are more fully set forth in the SEC Reports (as hereinafter defined).

4

6.2. Lack of Liquidity.  Each Purchaser confirms that he or it
is able  (i) to bear  the  economic  risk of this  investment,  (ii) to hold the
Notes,  the  Warrants  and any  shares of  Note-Underlying  Common  Stock for an
indefinite  period of time, and (iii) presently to afford a complete loss of his
or its investment;  and represents that he or it has sufficient liquid assets so
that the  illiquidity  associated  with this investment will not cause any undue
financial  difficulties or affect such Purchaser's ability to provide for his or
its current  needs and  possible  financial  contingencies,  and that his or its
commitment to all  speculative  investments  is reasonable in relation to his or
its net worth and annual income.  Furthermore,  each Purchaser acknowledges that
the Warrants contain certain  restrictions on exercise,  voting,  conversion and
certain other rights,  as more  particularly set forth in the Warrant  Agreement
attached hereto as Exhibit B.

6.3.   Knowledge  and   Experience.   Each  Purchaser   hereby
acknowledges and represents that such Purchaser has prior investment experience,
including investment in securities that are non-listed, unregistered and are not
traded  on  the  Nasdaq  National  or  SmallCap  Market,  nor  on  the  National
Association  of  Securities  Dealers,  Inc.'s (the "NASD")  automated  quotation
system,  or such  Purchaser  has  employed at its own expense the services of an
investment  advisor,  attorney and/or  accountant to request  documents from the
Company  pursuant  to  Section  6.5  hereof  and to  read  all of the  documents
furnished or made available by the Company to such Purchaser and to evaluate the
investment,  tax and  legal  merits  and the  consequences  and  risks of such a
transaction on such Purchaser's behalf, that such Purchaser or such professional
advisor has such knowledge and experience in financial and business  matters and
that such  Purchaser or such  professional  advisor is capable of evaluating the
merits and risks of the prospective investment and, if applicable, satisfies the
conditions set out in Rule 501(h) under the Securities Act.

6.4. Purchaser Capacity. Each Purchaser hereby represents that
such  Purchaser  either by  reason of such  Purchaser's  business  or  financial
experience,  or  the  business  or  financial  experience  of  such  Purchaser's
professional  advisors (who are  unaffiliated  with, and who are not compensated
by, the Company or any  affiliate or selling  agent of the Company,  directly or
indirectly),  has the  capacity to protect  such  Purchaser's  own  interests in
connection with the transaction contemplated hereby.

6.5.   Receipt   of   Information.   Each   Purchaser   hereby
acknowledges that such Purchaser has carefully  reviewed (a) the SEC Reports and
(b) this Agreement and all  attachments to it, and hereby  represents  that such
Purchaser  has  been  furnished  by  the  Company  during  the  course  of  this
transaction  with all information  regarding the Company which such Purchaser or
its  representative  has  requested  or desired to know,  has been  afforded the
opportunity to ask questions of, and to receive  answers from,  duly  authorized
officers  or other  representatives  of the  Company  concerning  the  terms and
conditions  of the  Offering,  the Notes,  the Warrants and the  Note-Underlying
Common  Stock and the  affairs of the Company and has  received  any  additional
information which such Purchaser or its representative has requested.

6.6. Reliance on Information. Each Purchaser has relied solely
upon the  information  provided  by the  Company in the SEC  Reports and in this
Agreement  in  making  the  decision  to  invest  in the  Notes.  To the  extent
necessary,  each Purchaser has retained,  at the sole expense of such Purchaser,
and relied upon, appropriate  professional advice regarding the investment,  tax
and legal  merits and  consequences  of this  Agreement  and its purchase of the

5

Notes, its potential acquisition of the Warrants and the conversion of the Notes
into, or exercise of the Warrants for, Note-Underlying Common Stock.

6.7. No Solicitation.  Each Purchaser represents that (i) such
Purchaser  was  contacted  regarding the sale of the Notes by the Company (or an
authorized agent or representative thereof) with whom such Purchaser had a prior
substantial pre-existing  relationship and (ii) no Notes were offered or sold to
such  Purchaser  by  means  of any  form  of  general  solicitation  or  general
advertising,  and in connection  therewith no Purchaser (A) received or reviewed
any  advertisement,  article,  notice  or  other  communication  published  in a
newspaper or magazine or similar  media or broadcast  over  television  or radio
whether  closed  circuit,  or generally  available;  or (B) attended any seminar
meeting or industry  investor  conference  whose  attendees  were invited by any
general solicitation or general advertising.

6.8. Registration. Each Purchaser hereby acknowledges that the
Offering has not been reviewed by the Securities and Exchange  Commission or any
state regulatory authority, since the Offering is intended to be exempt from the
registration  requirements  of  Section  5 of the  Securities  Act  pursuant  to
Regulation  D. No  Purchaser  shall sell or otherwise  transfer  the Notes,  the
Warrants  or  any  Note-Underlying  Common  Stock  unless  such  securities  are
registered   under  the   Securities  Act  or  unless  an  exemption  from  such
registration is available.

6.9. Purchase for own Account. Each Purchaser understands that
neither  the Notes nor the  Warrants  nor any shares of  Note-Underlying  Common
Stock  have  been  registered  under the  Securities  Act by reason of a claimed
exemption  under the provisions of the  Securities  Act which depends,  in part,
upon such Purchaser's investment intention.  In this connection,  each Purchaser
hereby  represents that such Purchaser is purchasing  Notes for such Purchaser's
own account for investment and not with a view toward the resale or distribution
to others or for resale in connection  with, any distribution or public offering
(within the meaning of the Securities  Act),  nor with any present  intention of
distributing  or  selling  the  same  and  such  Purchaser  has  no  present  or
contemplated  agreement,  undertaking,  arrangement,  obligation  or  commitment
providing for the disposition  thereof.  No Purchaser,  if an entity, was formed
for the purpose of purchasing the Notes.

6.10.  Holding  Period.  Nothing in this Section 6.10 shall be
construed  to relieve the Company of its  registration  obligations  pursuant to
Section 12, hereof.  Each Purchaser  understands  that there is no public market
for the Notes or the  Warrants and that no market is expected to develop for any
such Notes or Warrants.  Each Purchaser understands that even if a public market
develops for such Notes or Warrants, reliance upon Rule 144 under the Securities
Act for resales  requires,  among other  conditions,  a one-year  holding period
prior to the resale (in limited amounts) of securities  acquired in a non-public
offering  without  having to satisfy  the  registration  requirements  under the
Securities  Act. Each Purchaser  understands  and hereby  acknowledges  that the
Company is under no  obligation  to  register  any of the Notes or any  Warrants
under the Securities Act or any applicable  non-United States,  state securities
or "blue sky" laws.  Each  Purchaser  shall hold the Company and its  directors,
officers,  employees,  controlling  persons and agents and the Secured Party and
their respective heirs,  representatives,  successors and assigns harmless from,
and shall indemnify them against,  all liabilities,  costs and expenses incurred
by  them  as a  result  of (i) any  misrepresentation  made  by  such  Purchaser
contained in this Agreement  (including in Article 14 hereof),  (ii) any sale or
distribution  by  such

6

Purchaser  in  violation  of the  Securities  Act or any  applicable  non-United
States,  state  securities or "blue sky" laws or (iii) any untrue statement made
by such Purchaser.

6.11. Legends. Each Purchaser consents to the placement of the
legend set forth  below on any  certificate  or other  document  evidencing  the
Notes:

THE  TERMS  OF  THIS  NOTE  ARE  SUBJECT  TO  THE  TERMS  OF A
SUBSCRIPTION AGREEMENT AND AN INTERCREDITOR AGREEMENT,  COPIES
OF WHICH ARE AVAILABLE  FROM HYBRIDON,  INC. (THE  "COMPANY").
THE  SECURITIES   REPRESENTED  BY  THIS  NOTE  HAVE  NOT  BEEN
REGISTERED UNDER THE UNITED STATES  SECURITIES ACT OF 1933, AS
AMENDED  (THE  "SECURITIES  ACT"),  OR  ANY  APPLICABLE  STATE
SECURITIES  LAWS,  AND  MAY NOT BE  SOLD,  OFFERED  FOR  SALE,
PLEDGED  OR  HYPOTHECATED  OR  OTHERWISE  TRANSFERRED  IN  THE
ABSENCE OF A REGISTRATION  STATEMENT IN EFFECT WITH RESPECT TO
THE  SECURITIES  UNDER THE SECURITIES ACT OR AN EXEMPTION FROM
THE  SECURITIES  ACT. ANY SUCH TRANSFER MAY ALSO BE SUBJECT TO
COMPLIANCE WITH APPLICABLE  STATE SECURITIES LAWS AND THE LAWS
OF OTHER APPLICABLE JURISDICTIONS.

THE SECURED PARTY (AS DEFINED IN THE  SUBSCRIPTION  AGREEMENT)
IS THE EXCLUSIVE AGENT OF THE HOLDER OF THIS NOTE WITH RESPECT
TO  CERTAIN  ACTIONS  HEREUNDER  AND  UNDER  THE  SUBSCRIPTION
AGREEMENT; THE SECURED PARTY, IN HIS SOLE DISCRETION, MAY TAKE
OR FOREBEAR FROM TAKING  CERTAIN  ACTIONS  HEREUNDER AND UNDER
THE SUBSCRIPTION AGREEMENT ON BEHALF OF THE HOLDERS OF NOTES.

Each  Purchaser  further  consents to the  placement of one or more  restrictive
legends on the  Warrants  and the  Note-Underlying  Common  Stock as required by
applicable securities laws. Each Purchaser is aware that the Company will make a
notation in its  appropriate  records  with respect to the  restrictions  on the
transferability of such Notes, Warrants and Note-Underlying Common Stock.

6.12.  Financial Review.  Each Purchaser  understands that the
Company  will  review  this  Agreement  and is hereby  given  authority  by each
Purchaser  to call such  Purchaser's  bank or place of  employment  or otherwise
review the financial  standing of such Purchaser;  and it is further agreed that
the Company, at its sole discretion,  reserves the unrestricted  right,  without

7

further  documentation or agreement on the part of such Purchaser,  to reject or
limit any purchase, and to close the Offering to such Purchaser at any time.

6.13. Residence of Purchaser. Each Purchaser hereby represents
that the address of such Purchaser  furnished by such Purchaser on the signature
page hereof is such  Purchaser's  principal  residence  if such  Purchaser is an
individual or its  principal  business  address if it is a corporation  or other
entity.

6.14. Power and Authority. Each Purchaser represents that such
Purchaser has full power and authority  (corporate,  statutory and otherwise) to
execute and deliver this  Agreement and to purchase the Notes,  the Warrants and
any shares of  Note-Underlying  Common Stock.  This  Agreement  constitutes  the
legal, valid and binding obligation of each Purchaser,  enforceable against such
Purchaser in accordance with its terms.

6.15.  Plans.  If a Purchaser is a  corporation,  partnership,
limited liability company,  trust,  employee benefit plan, individual retirement
account or other entity,  then subject to the terms  contained in this Agreement
(a) it is authorized  and qualified to become an investor in the Company and the
person signing this Agreement on behalf of such entity has been duly  authorized
by such entity to do so, and (b) it is duly organized,  validly  existing and in
good standing under the laws of the jurisdiction of its organization.

6.16. NASD. Each Purchaser acknowledges that if he or she is a
registered  representative of an NASD member firm, he or she must give such firm
the notice required by the NASD's Rules of Fair Practice,  receipt of which must
be acknowledged by such firm in Section 14.3 below.

6.17.  Securities  Laws. Each Purchaser  acknowledges  that at
such time, if ever, as the Notes, the Warrants or  Note-Underlying  Common Stock
are  registered,  sales of the Notes,  the Warrants and  Note-Underlying  Common
Stock will be subject to applicable non-United States and state securities laws,
including,  without  limitation,  those of the State of New Jersey which require
any securities sold in New Jersey to be sold through a registered  broker-dealer
or in reliance upon an exemption from registration.

6.18. Brokers.  Each Purchaser represents and warrants that it
has not engaged,  consented to nor authorized any broker, finder or intermediary
to  act  on  its  behalf,  directly  or  indirectly,  as  a  broker,  finder  or
intermediary in connection with the transactions contemplated by this Agreement.
Each  Purchaser  shall  indemnify and hold harmless the Company from and against
all fees,  commissions or other payments owing to any such person or firm acting
on behalf of such Purchaser hereunder.

6.19. Recent Financing Activities. Each Purchaser acknowledges
that it is aware of the  following  recent  financing  activities of the Company
that have not yet been disclosed in the SEC Reports, and that such Purchaser has
had the opportunity to review the agreements and instruments  relating  thereto,
and to ask  questions  of the Company  regarding  the same:  (a) an aggregate of
$1,000,000  principal  amount  of  promissory  notes  were  sold  to E.  Andrews
Grinstead III, who is the Company's  President and Chief Executive  Officer,  at
face value during September 1999; (b) an additional $500,000 principal amount of
promissory  notes

8

were sold to E. Andrews  Grinstead III, at face value during  November 1999; and
(c) an aggregate of  approximately  $455,000 of debt was sold to purchasers in a
private placement  transaction in October 1999, which debt will, on December 31,
1999,  automatically convert into either Notes or preferred stock of the Company
having the terms set forth in the term sheet attached hereto as Exhibit E.

6.20. Beneficial Owner. Each Purchaser,  whose name appears on
the signature line below,  will be the  beneficial  owner of the Notes that such
Purchaser acquires.

6.21.  Accredited Investor.  Each Purchaser represents that it
is an "accredited investor" as such term is defined in Rule 501 of Regulation D.

6.22.   Reliance  on  Representation   and  Warranties.   Each
Purchaser  understands  that  the  Notes  are  being  offered  and  sold  to the
undersigned   in  reliance  on  specific   exemptions   from  the   registration
requirements  of United States  Federal and state  securities  laws and that the
Company  is  relying  upon  the  truth  and  accuracy  of  the  representations,
warranties,  agreements,  acknowledgments  and understandings of the undersigned
set forth herein in order to determine the  applicability of such exemptions and
the  suitability of the  undersigned to acquire the Notes,  the Warrants and the
Note-Underlying Common Stock.

6.23.    Reserved.

6.24.  Abdication of Rights to Secured  Party.  Each Purchaser
acknowledges that such Purchaser has irrevocably designated Secured Party to act
on such  Purchaser's  behalf with  respect to the Notes,  the  Warrants  and the
Collateral,  under the Notes,  the Warrant  Agreement and this  Agreement.  Each
Purchaser further  acknowledges and accepts that the actions of Secured Party on
such  Purchaser's  behalf may be materially  different  than how such  Purchaser
would have acted in such  Purchaser's own capacity,  and that such Purchaser may
be materially and adversely affected thereby.

7.       [Reserved]

8.  Representation,  Warranties  and Covenants of the Company.
The Company hereby represents, warrants and covenants to each Purchaser that all
reports  required to be filed by the Company since and including the most recent
filing  of the  Company's  Annual  Report on Form  10-K,  to and  including  the
relevant  Closing Date  (collectively,  the "SEC  Reports") have been duly filed
with the Securities and Exchange  Commission,  complied at the time of filing in
all material  respects with the  requirements of their respective forms and were
complete  and  correct  in all  material  respects  as of the dates at which the
information was furnished,  and contained (as of such dates) no untrue statement
of a material  fact or omitted to state a material  fact  necessary  in order to
make the statements contained therein, in light of the circumstances under which
they were made, not misleading.

9.       [Reserved]

10.      Secured Party's Rights and Remedies

9

10.1. Rights and Remedies.  Upon the occurrence and during the
continuance  of an Event of Default (as defined in the Note),  the Secured Party
may,  in  addition to the  remedies  pursuant to Section 6 of the Notes,  at its
election,  without  notice of its election and without  demand,  take any action
permitted  by law,  including  the  exercise  of any  rights  accorded a secured
creditor under the Uniform  Commercial Code as in effect in the  Commonwealth of
Massachusetts at such time.

11.  Certain  Definitions.  For the purposes of this Agreement
the following terms have the respective meanings set forth below:

11.1.  "Business  Day" means a Monday  through Friday on which
banks are generally open for business in New York, Massachusetts and California.

11.2.  "Common  Stock" means the Company's  common stock,  par
value $.001 per share.

11.3.  "Note-Underlying  Common  Stock"  shall mean the Common
Stock issuable upon conversion of, or in lieu of cash interest on, the Notes and
any Common Stock  issuable upon exercise of any Warrants that may be issued upon
prepayment of Notes.

11.4. "Offering" shall have the meaning ascribed to such terms
in the first paragraph of this Agreement.

11.5.  "Person"  means any  individual,  sole  proprietorship,
partnership,  limited liability company,  joint venture,  trust,  unincorporated
organization, association, corporation, institution, public benefit corporation,
firm, joint stock company, estate, entity or governmental agency.

11.6.  "Regulation D" means Regulation D promulgated under the
Securities Act.

11.7.  "Secured Party" shall have the meaning ascribed to such
term in Section 5.1.

11.8.  "Securities  Act"  means,  as of any  given  time,  the
Securities Act of 1933, as amended, or any similar federal law then in force.

11.9.   "Securities  and  Exchange  Commission"  includes  any
governmental body or agency succeeding to the functions thereof.

11.10.  "Operating  Indebtedness"  means the principal of (and
premium,  if any) and  accrued  interest on  (including  all  interest  accruing
subsequent to the commencement of any bankruptcy or similar proceeding,  whether
or not a claim for  post-petition  interest is  allowable as a claim in any such
proceeding) all reimbursement  obligations and other liabilities  (contingent or
otherwise)  with  respect  to letters of credit,  bank  guarantees  or  bankers'
acceptances,  and  any  amendments,  renewals,  extensions,   modifications  and
refundings  of  any  such  indebtedness  or  obligation.   Notwithstanding   the
foregoing,  the Operating Indebtedness shall not include any

10

such  obligations  or  liabilities  to the extent they exceed  $1,000,000 in the
aggregate at any time outstanding.

11.11.  "SEC Reports" shall have the meaning  ascribed thereto
in Article 8.

11.12.  "Transfer  Restricted  Securities" means each Warrant,
each Note and, if any Note has been converted or any Warrant has been exercised,
the Note-Underlying Common Stock issued upon such conversion or exercise,  until
the  earlier  of (a) the date on which such  Note,  Warrant  or  Note-Underlying
Common  Stock,  as  applicable,   has  been  effectively  registered  under  the
Securities Act and disposed of pursuant to, and in accordance with, an effective
registration  statement  under the  Securities  Act,  (b) the date on which such
Note, Warrant or Note-Underlying Common Stock, as applicable,  is distributed to
the public  pursuant  to Rule 144 or any other  applicable  exemption  under the
Securities Act without additional  restriction upon public resale or (c) at such
time as such Note, Warrant or Note-Underlying  Common Stock, as applicable,  may
be sold by a Holder under Rule 144(k).

12.      Registration Rights.

12.1 As used in this  Article  12, the  following  terms shall
have the following meanings:

(a)  "Affiliate"  shall mean,  with  respect to any Person (as
defined below), any other Person controlling,  controlled by, or under direct or
indirect  common control with,  such Person (for the purposes of this definition
"control," when used with respect to any specified Person,  shall mean the power
to direct the  management  and policies of such person,  directly or indirectly,
whether through ownership of voting  securities,  by contract or otherwise;  and
the terms "controlling" and "controlled" shall have meanings  correlative to the
foregoing).

(b)      [Reserved].

(c) "Holders" shall mean the Purchasers and any person holding
Registrable  Securities  or any person to whom the rights under  Article 12 have
been transferred in accordance with Section 12.10 hereof.

(d)      [Reserved]

(e) The  terms  "register,"  "registered"  and  "registration"
refer to the  registration  effected  by  preparing  and  filing a  registration
statement in compliance with the Securities Act, and the declaration or ordering
of the effectiveness of such registration statement.

(f) "Registrable  Securities"  shall mean the shares of Common
Stock issuable upon conversion of the Notes; provided,  however, that securities
shall only be treated as Registrable  Securities if and only for so long as they
(A) have not been  disposed  of pursuant to a  registration  statement  declared
effective by the Securities and Exchange Commission, (B) have not been sold in a
transaction exempt from the registration and prospectus delivery requirements of
the Securities Act so that all transfer  restrictions  and  restrictive  legends
with respect  thereto are removed upon the  consummation  of such sale,  (C) are
held by a Holder or a permitted

11

transferee  pursuant to Section  12.10 or (D) are not freely  tradeable  without
limitations  as to  volume  or  filing  requirements  under  applicable  federal
securities laws.

(g)  "Registration  Expenses" shall mean all expenses incurred
by the  Company in  complying  with  Section  12.2  hereof,  including,  without
limitation, all registration,  qualification and filing fees, printing expenses,
escrow fees,  fees and  expenses of counsel for the  Company,  blue sky fees and
expenses and the expense of any special audits  incident to, or required by, any
such registration (but excluding the fees of legal counsel for any Holder).

(h)  "Registration  Statement" shall have the meaning ascribed
to such term in Section 12.2.

(i)  "Registration  Period" shall have the meaning ascribed to
such term in Section 12.4.

(j) "Selling  Expenses" shall mean all underwriting  discounts
and selling commissions applicable to the sale of Registrable Securities and all
fees and expenses of legal counsel for any Holder.

12.2.  The Company  shall use its  reasonable  best efforts to
file a "shelf" registration statement on the appropriate form (the "Registration
Statement")  with the  Securities and Exchange  Commission,  by ninety (90) days
after the  Termination  Date and shall use its reasonable best efforts to effect
the registration,  qualifications or compliances (including, without limitation,
the execution of any required  undertaking  to file  post-effective  amendments,
appropriate  qualifications  or exemptions  under  applicable  blue sky or other
state  securities  laws and appropriate  compliance  with applicable  securities
laws,  requirements or  regulations)  prior to the date which is two hundred ten
(210)  days after the  Termination  Date.  Notwithstanding  the  foregoing,  the
Company shall not be obligated to enter into any underwriting  agreement for the
sale of any of the Registrable Securities.

12.3. All  Registration  Expenses  incurred in connection with
any registration,  qualification or compliance pursuant to Section 12.2 shall be
borne by the Company.  All Selling  Expenses  relating to the sale of securities
registered by or on behalf of Holders shall be borne by such Holders pro rata on
the basis of the number of securities so  registered;  provided that if a Holder
uses its own legal  counsel in addition to one counsel for all of the Holders of
securities  registered on behalf of the Holders, such Holder shall bear the cost
of such counsel.

12.4.  In the  case  of  the  registration,  qualification  or
compliance  effected by the  Company  pursuant  to this  Agreement,  the Company
shall,  upon  reasonable  request,  inform  each Holder as to the status of such
registration, qualification and compliance. At its expense the Company shall:

(a) use its reasonable best efforts to keep such registration,
and any qualification, exemption or compliance under state securities laws which
the Company determines to obtain,  continuously effective until the Holders have
completed the  distribution  described in the  registration  statement  relating
thereto.  Notwithstanding the foregoing,  at the Company's election, the Company
may cease to keep such registration,  qualification or compliance effective with
respect to any Registrable  Securities,  and the registration rights of a

12

Holder  shall  expire,  upon the earlier of (i) such time as the Holder may sell
under Rule 144(k) under the Securities Act (or other exemption from registration
acceptable to the Company) in a three-month  period all  Registrable  Securities
then held by such Holder and (ii)  November 30, 2002.  The period of time during
which the  Company is  required  hereunder  to keep the  Registration  Statement
effective is referred to herein as the "Registration Period."; and

(b)      advise the Holders:

(i) when the Registration  Statement or any amendment  thereto
has  been  filed  with  the  Securities  and  Exchange  Commission  and when the
Registration  Statement  or any  post-effective  amendment  thereto  has  become
effective;

(ii) of any request by the Securities and Exchange  Commission
for amendments or supplements  to the  Registration  Statement or the prospectus
included therein or for additional information;

(iii)  of  the  issuance  by  the   Securities   and  Exchange
Commission of any stop order  suspending the  effectiveness  of the Registration
Statement or the initiation of any proceedings for such purpose;

(iv) of the  receipt by the Company of any  notification  with
respect to the suspension of the  qualification  of the  Registrable  Securities
included  therein for sale in any  jurisdiction or the initiation or threatening
of any proceeding for such purpose; and

(v) of the  happening of any event that requires the making of
any changes in the Registration  Statement or the prospectus included therein so
that, as of such date, the statements therein are not misleading and do not omit
to state a material fact required to be stated  therein or necessary to make the
statements  therein  (in  the  case  of  the  prospectus,  in the  light  of the
circumstances under which they were made) not misleading;

(vi) make every reasonable  effort to obtain the withdrawal of
any order  suspending the  effectiveness  of any  Registration  Statement at the
earliest possible time;

(vii)  furnish to each Holder,  without  charge,  at least one
copy of such Registration  Statement and any  post-effective  amendment thereto,
including financial statements and schedules,  and, if the Holder so requests in
writing,  all exhibits  (including those  incorporated by reference) in the form
filed with the Securities and Exchange Commission;

(viii) during the Registration Period, deliver to each Holder,
without charge,  as many copies of the prospectus  included in such Registration
Statement and any amendment or supplement  thereto as such Holder may reasonably
request;  and the Company  consents to the use,  consistent  with the provisions
hereof, of the prospectus or any amendment or supplement  thereto by each of the
selling  Holders of Registrable  Securities in connection  with the offering and
sale of the Registrable  Securities  covered by the prospectus and any amendment
or supplement  thereto.  In addition,  upon the reasonable request of the Holder
and subject in all cases to confidentiality protections reasonably acceptable to
the Company, the Company will meet with a Holder or a representative  thereof at
the Company's headquarters to discuss all information relevant for disclosure in
the  Registration  Statement  covering  the  Registrable

13

Securities,   and  will  otherwise  cooperate  with  any  Holder  conducting  an
investigation  for the purpose of reducing or eliminating such Holder's exposure
to liability  under the Securities Act,  including the reasonable  production of
information at the Company's headquarters;

(ix) during the Registration  Period,  deliver to each Holder,
without charge, (i) as soon as practicable (but in the case of the annual report
of the Company to its stockholders, within 120 days after the end of each fiscal
year of the Company) one copy of: (A) its annual report to its stockholders,  if
any  (which  annual  report  shall  contain  financial   statements  audited  in
accordance with generally accepted accounting principles in the United States of
America by a firm of certified public accountants of recognized  standing);  (B)
if not included in substance in its annual  report to  stockholders,  its annual
report on Form 10-K (or similar form); (C) each of its quarterly  reports to its
stockholders,  and, if not  included in substance  in its  quarterly  reports to
stockholders,  its quarterly  report on Form 10-Q (or similar  form),  and (D) a
copy of the full Registration Statement (the foregoing,  in each case, excluding
exhibits);  and (ii) upon  reasonable  request,  all  exhibits  excluded  by the
parenthetical to the immediately preceding clause (D), and all other information
that is generally available to the public;

(x) prior to any public  offering  of  Registrable  Securities
pursuant to any Registration  Statement,  register or qualify for offer and sale
under the securities or blue sky laws of such  jurisdictions as any such Holders
reasonably request in writing,  provided that the Company shall not for any such
purpose be  required  to qualify  generally  to  transact  business as a foreign
corporation  in any  jurisdiction  where it is not so qualified or to consent to
general  service of process in any such  jurisdiction,  and do any and all other
acts or things reasonably necessary or advisable to enable the offer and sale in
such  jurisdictions of the Registrable  Securities  covered by such Registration
Statement;

(xi)  cooperate  with the  Holders  to  facilitate  the timely
preparation and delivery of certificates  representing Registrable Securities to
be sold pursuant to any Registration  Statement free of any restrictive  legends
to the extent not required at such time and in such denominations and registered
in such names as Holders may request at least three (3)  business  days prior to
sales of Registrable Securities pursuant to such Registration Statement;

(xii) upon the occurrence of any event contemplated by Section
12.4(b)(v) above, the Company shall promptly prepare a post-effective  amendment
to the Registration Statement or a supplement to the related prospectus, or file
any other  required  document so that, as thereafter  delivered to purchasers of
the Registrable Securities included therein, the prospectus will not include any
untrue statement of a material fact or omit to state any material fact necessary
to make the statements  therein,  in the light of the circumstances  under which
they were made, not misleading; and

(xiii) use its  reasonable  best  efforts  to comply  with all
applicable rules and regulations of the Securities and Exchange Commission,  and
will make generally  available to the Holders not later than 45 days (or 90 days
if the fiscal quarter is the fourth fiscal  quarter) after the end of its fiscal
quarter  in  which  the  first  anniversary  date of the  effective  date of the
Registration  Statement occurs, an earnings statement  satisfying the provisions
of Section 11(a) of the Securities Act.

14

12.5.  Delay Periods;  Suspension of Sales.  Each Holder shall
suspend, upon request of the Company, any disposition of Registrable  Securities
pursuant to the  Registration  Statement and prospectus  contemplated by Section
12.2  during (i) any period  not to exceed  two  30-day  periods  within any one
12-month period the Company  requires in connection with a primary  underwritten
offering  of equity  securities  and (ii) any  period,  not to exceed one 45-day
period per  circumstance  or  development,  when the Company  determines in good
faith that offers and sales pursuant thereto should not be made by reason of the
presence of material  undisclosed  circumstances or developments with respect to
which the  disclosure  that would be required in such a prospectus is premature,
would  have an  adverse  effect  on the  Company  or is  otherwise  inadvisable;
provided  that the Company  makes such  determination  and applies such halts of
offers and sales  uniformly and  universally to all Persons then offering shares
of  Common  Stock  pursuant  to  effective  Registration  Statements  (including
Registration Statements on Forms S-4 and S-8).

12.6.  The  Holders  shall have no right to take any action to
restrain,  enjoin or otherwise delay any  registration  pursuant to Section 12.2
hereof  as a result  of any  controversy  that may  arise  with  respect  to the
interpretation or implementation of this Agreement.

12.7.  (a) To the extent  permitted by law, the Company  shall
indemnify each Holder,  each underwriter of the Registrable  Securities and each
person  controlling  such  Holder  within  the  meaning  of  Section  15 of  the
Securities  Act,  with  respect  to which  any  registration,  qualification  or
compliance  has been effected  pursuant to this  Agreement,  against all claims,
losses, damages and liabilities (or action in respect thereof), including any of
the foregoing incurred in settlement of any litigation,  commenced or threatened
(subject to  Subsection  12.7(c)  below),  arising out of or based on any untrue
statement (or alleged  untrue  statement)  of a material  fact  contained in any
registration  statement,  prospectus or offering  circular,  or any amendment or
supplement  thereof,  incident  to  any  such  registration,   qualification  or
compliance,  or based on any omission (or alleged  omission) to state  therein a
material fact required to be stated  therein or necessary to make the statements
therein not misleading,  in light of the  circumstances in which they were made,
and shall reimburse each Holder, each underwriter of the Registrable  Securities
and each person controlling such Holder, for legal and other expenses reasonably
incurred in connection  with  investigating  or defending any such claim,  loss,
damage, liability or action as incurred;  provided that the Company shall not be
liable in any such case to the extent that any untrue  statement  or omission or
allegation  thereof is made in reliance upon and in conformity with  information
furnished  to the  Company  by or on  behalf  of such  Holder  and  stated to be
specifically for use in preparation of such registration  statement,  prospectus
or offering circular;  provided that the Company shall not be liable in any such
case where the claim,  loss,  damage or liability arises out of or is related to
the failure of the Holder to comply with the covenants and agreements  contained
in this Agreement  respecting sales of Registrable  Securities,  and except that
the foregoing  indemnity  agreement is subject to the condition that, insofar as
it relates to any such untrue  statement or alleged untrue statement or omission
or  alleged  omission  made in the  preliminary  prospectus  but  eliminated  or
remedied in the amended  prospectus  on file with the  Securities  and  Exchange
Commission at the time the registration  statement  becomes  effective or in the
amended prospectus filed with the Securities and Exchange Commission pursuant to
Rule 424(b) of the Securities Act or in the prospectus subject to completion and
term  sheet  under  Rule 434 of the  Securities  Act,  which  together  meet the
requirements  of Section 10(a) of the Securities  Act (the "Final  Prospectus"),
such indemnity  agreement shall not inure to

15

the benefit of any such Holder,  any such  underwriter  or any such  controlling
person, if a copy of the Final Prospectus furnished by the Company to the Holder
for  delivery  was not  furnished  to the person or entity  asserting  the loss,
liability,  claim or damage at or prior to the time such  furnishing is required
by the  Securities  Act and the Final  Prospectus  would  have  cured the defect
giving rise to such loss, liability, claim or damage.

(b) Each Holder will severally, if Registrable Securities held
by such Holder are  included in the  securities  as to which such  registration,
qualification  or compliance is being effected,  indemnify the Company,  each of
its directors and officers,  each underwriter of the Registrable  Securities and
each person who  controls  the  Company  within the meaning of Section 15 of the
Securities Act, against all claims,  losses, damages and liabilities (or actions
in respect  thereof),  including any of the foregoing  incurred in settlement of
any litigation,  commenced or threatened  (subject to Subsection 12.7(c) below),
arising out of or based on any untrue statement (or alleged untrue statement) of
a material fact contained in any registration statement,  prospectus or offering
circular,  or  any  amendment  or  supplement  thereof,  incident  to  any  such
registration,  qualification or compliance, or based on any omission (or alleged
omission)  to state  therein a material  fact  required to be stated  therein or
necessary  to make  the  statements  therein  not  misleading,  in  light of the
circumstances  in which they were made,  and will  reimburse  the Company,  such
directors and officers,  each underwriter of the Registrable Securities and each
person  controlling  the Company  for  reasonable  legal and any other  expenses
reasonably  incurred in  connection  with  investigating  or defending  any such
claim,  loss,  damage,  liability  or  action as  incurred,  in each case to the
extent,  but only to the  extent,  that such  untrue  statement  or  omission or
allegation  thereof is made in  reliance  upon and in  conformity  with  written
information furnished to the Company by or on behalf of the Holder and stated to
be  specifically  for  use  in  preparation  of  such  registration   statement,
prospectus or offering circular;  provided that the indemnity shall not apply to
the extent that such claim, loss, damage or liability results from the fact that
a current copy of the  prospectus  was not made available to the Holder and such
current copy of the  prospectus  would have cured the defect giving rise to such
loss, claim,  damage or liability.  Notwithstanding  the foregoing,  in no event
shall a Holder be liable for any such claims,  losses, damages or liabilities in
excess of the proceeds  received by such Holder in the  offering,  except in the
event of fraud by such Holder.

(c) Each party entitled to indemnification  under this Section
12.7 (the  "Indemnified  Party")  shall  give  notice to the party  required  to
provide   indemnification   (the  "Indemnifying   Party")  promptly  after  such
Indemnified Party has actual knowledge of any claim as to which indemnity may be
sought,  and shall  permit the  Indemnifying  Party to assume the defense of any
such claim or any litigation resulting therefrom,  provided that counsel for the
Indemnifying  Party,  who shall conduct the defense of such claim or litigation,
shall  be  approved  by  the   Indemnified   Party  (whose  approval  shall  not
unreasonably  be withheld),  and the  Indemnified  Party may participate in such
defense at such  Indemnified  Party's  expense,  and  provided  further that the
failure of any  Indemnified  Party to give notice as provided  herein  shall not
relieve the Indemnifying  Party of its obligations under this Agreement,  unless
such failure is materially  prejudicial to the  Indemnifying  Party in defending
such claim or  litigation.  An  Indemnifying  Party  shall not be liable for any
settlement of an action or claim  effected  without its written  consent  (which
consent will not be unreasonably withheld).

16

(d) If the  indemnification  provided for in this Section 12.7
is held by a court of competent jurisdiction to be unavailable to an Indemnified
Party with respect to any loss, liability,  claim, damage or expense referred to
therein,  then the Indemnifying  Party, in lieu of indemnifying such Indemnified
Party  thereunder,  shall  contribute  to the  amount  paid or  payable  by such
Indemnified Party as a result of such loss, liability,  claim, damage or expense
in such  proportion  as is  appropriate  to reflect  the  relative  fault of the
Indemnifying  Party on the one hand and of the Indemnified Party on the other in
connection  with the  statements  or  omissions  which  resulted  in such  loss,
liability,  claim,  damage or  expense as well as any other  relevant  equitable
considerations.  The  relative  fault  of  the  Indemnifying  Party  and  of the
Indemnified  Party shall be  determined  by reference  to,  among other  things,
whether  the  untrue or  alleged  untrue  statement  of a  material  fact or the
omission  to state a  material  fact  relates  to  information  supplied  by the
Indemnifying Party or by the Indemnified Party and the parties' relative intent,
knowledge,  access to  information  and  opportunity  to correct or prevent such
statement or omission.

12.8. (a) Each Holder agrees that,  upon receipt of any notice
from the Company of the happening of any event  requiring the  preparation  of a
supplement or amendment to a prospectus  relating to  Registrable  Securities so
that, as thereafter  delivered to the Holders,  such prospectus will not contain
an  untrue  statement  of a  material  fact or omit to state any  material  fact
required to be stated  therein or necessary to make the  statements  therein not
misleading,  each Holder will forthwith  discontinue  disposition of Registrable
Securities pursuant to the registration  statement  contemplated by Section 12.2
until its receipt of copies of the  supplemented or amended  prospectus from the
Company  and, if so directed by the Company,  each Holder  shall  deliver to the
Company  all copies,  other than  permanent  file  copies then in such  Holder's
possession,  of the prospectus  covering such Registrable  Securities current at
the time of receipt of such notice.

(b)  As a  condition  to  the  inclusion  of  its  Registrable
Securities,  each Holder shall furnish to the Company such information regarding
such  Holder and the  distribution  proposed  by such  Holder as the Company may
request in writing or as shall be required in connection with any  registration,
qualification or compliance referred to in this Article 12.

(c) Each Holder hereby  covenants  with the Company (i) not to
make any sale of the  Registrable  Securities  without  effectively  causing the
prospectus delivery  requirements under the Securities Act to be satisfied,  and
(ii) if such  Registrable  Securities  are to be  sold by any  method  or in any
transaction other than on a national  securities  exchange,  the Nasdaq National
Market,  Nasdaq SmallCap Market or in the over-the-counter  market, in privately
negotiated  transactions,  or in a combination  of such  methods,  to notify the
Company  at least five (5)  business  days prior to the date on which the Holder
first offers to sell any such Registrable Securities.

(d) Each Holder  acknowledges  and agrees that the Registrable
Securities sold pursuant to the Registration Statement described in this Article
12 are not transferable on the books of the Company unless the stock certificate
submitted  to the transfer  agent  evidencing  such  Registrable  Securities  is
accompanied  by a  certificate  reasonably  satisfactory  to the  Company to the
effect that (i) the  Registrable  Securities  have been sold in accordance  with
such

17

Registration  Statement  and  (ii)  the  requirement  of  delivering  a  current
prospectus has been satisfied.

(e) Each Holder  shall not take any action with respect to any
distribution  deemed to be made pursuant to such registration  statement,  which
would  constitute a violation of Regulation M under the Securities  Exchange Act
of  1934,  as  amended  (the  "Exchange  Act")  or any  other  applicable  rule,
regulation or law.

(f) At the end of the  Registration  Period,  the  Holders  of
Registrable  Securities included in the Registration Statement shall discontinue
sales of shares pursuant to such  Registration  Statement upon receipt of notice
from the Company of its intention to remove from registration the shares covered
by such  Registration  Statement  which remain  unsold,  and such Holders  shall
notify the  Company  of the  number of shares  registered  which  remain  unsold
immediately upon receipt of such notice from the Company.

12.9.  With a view to  making  available  to the  Holders  the
benefits  of  certain  rules and  regulations  of the  Securities  and  Exchange
Commission  which at any time permit the sale of the  Registrable  Securities to
the public  without  registration,  the Company  shall use its  reasonable  best
efforts:

(a) to make and keep public  information  available,  as those
terms are understood  and defined in Rule 144 under the  Securities  Act, at all
times;

(b) to file with the Securities  and Exchange  Commission in a
timely manner all reports and other documents  required of the Company under the
Exchange Act; and

(c) so long as a  Holder  owns  any  unregistered  Registrable
Securities,  to furnish to such  Holder  upon any  reasonable  request a written
statement by the Company as to its compliance with Rule 144 under the Securities
Act,  and of the  Exchange  Act, a copy of the most recent  annual or  quarterly
report of the Company,  and such other  reports and  documents of the Company as
such Holder may reasonably  request in availing itself of any rule or regulation
of the  Securities  and Exchange  Commission  allowing a Holder to sell any such
securities without registration.

12.10. The rights to cause the Company to register Registrable
Securities  granted to the  Holders by the  Company  under  Section  12.2 may be
assigned in full by a Holder in connection with a transfer by such Holder of its
Registrable  Securities,  provided  that  (i) such  transfer  may  otherwise  be
effected in  accordance  with  applicable  securities  laws;  (ii) such transfer
involves  not  fewer  than the fewer of all or  20,000  shares of such  Holder's
Registrable  Securities,  (iii) such Holder  gives prior  written  notice to the
Company; and (iv) such transferee agrees to comply with the terms and provisions
of this  Agreement,  and such  transfer is  otherwise  in  compliance  with this
Agreement. Except as specifically permitted by this Section 12.10, the rights of
a Holder with respect to  Registrable  Securities as set out herein shall not be
transferable  to any other Person,  and any attempted  transfer  shall cause all
rights of such Holder therein to be forfeited.

12.11. With the written consent of the Company and the Secured
Party, any provision of this Article 12 may be waived (either

18

generally or in a particular instance, either retroactively or prospectively and
either for a specified  period of time or  indefinitely)  or  amended.  Upon the
effectuation  of each such waiver or amendment,  the Company shall promptly give
written notice thereof to the Holders,  if any, who have not previously received
notice thereof.

13.      Miscellaneous.

13.1.  Amendments  and  Waivers.  (a) This  Agreement  and all
exhibits and schedules hereto set forth the entire  agreement and  understanding
among the parties as to the subject  matter hereof and merges and supersedes all
prior  discussions,  agreements and understandings of any and every nature among
them.  This  Agreement  may be amended only by mutual  written  agreement of the
Company  and the  Secured  Party,  and the  Company  may take any action  herein
prohibited or omit to take any action herein required to be performed by it, and
any breach of any covenant, agreement, warranty or representation may be waived,
only if the Company has  obtained  the written  consent or waiver of the Secured
Party.  No course of dealing between or among any persons having any interest in
this Agreement will be deemed  effective to modify,  amend or discharge any part
of this  Agreement or any rights or obligations of any person under or by reason
of this Agreement.

(b) After an  amendment or waiver  becomes  effective it shall
bind every holder of a Note  regardless of whether such holder held such Note at
the time such amendment or waiver became  effective,  or  subsequently  acquired
such Note.

13.2.  Successors and Assigns. This Agreement shall be binding
upon and inure to the benefit of the Company and its  successors and assigns and
Purchasers and their successors and registered  assigns.  The provisions  hereof
which are for Purchasers' benefit as purchasers or holders of the Notes are also
for the benefit of, and enforceable by, any subsequent registered holder of such
Notes.

13.3. Notices.  All notices,  demands and other communications
to be given or delivered  under or by reason of the provisions of this Agreement
shall be in writing  and shall be deemed to have been given  personally  or when
mailed by certified or registered  mail,  return  receipt  requested and postage
prepaid,  and  addressed to the  addresses of the  respective  parties set forth
below or to such  changed  addresses  as such  parties may have fixed by notice;
provided,  however, that any notice of change of address shall be effective only
upon receipt:

If to the Company:

Hybridon, Inc.
155 Fortune Boulevard
Milford, MA  01757
Attn: E. Andrews Grinstead, III

If to Secured Party or Purchasers:

c/o Pillar
28 Avenue de Messine
75008 Paris, France; and

19

c/o Pecks Management
1 Rockefeller Plaza, Suite 900
Attn: Art Berry
New York, NY  10020; and

c/o Forum Capital Markets
53 Forest Avenue
Old Greenwich, CT  06870
Attn: Hal Purkey

13.4. Governing Law. The validity,  performance,  construction
and effect of this Agreement shall be governed by the internal laws of the State
of Massachusetts without giving effect to such State's principles of conflict of
laws.

13.5.  Counterparts.  This  Agreement  may be  executed in any
number of  counterparts  and,  notwithstanding  that any of the  parties did not
execute the same  counterpart,  each of such  counterparts  (or facsimile copies
thereof)  shall,  for  all  purposes,  be  deemed  an  original,  and  all  such
counterparts  shall constitute one and the same instrument binding on all of the
parties hereto.  Delivery of an executed counterpart of a signature page to this
Agreement by telecopier shall be as effective as delivery of a manually executed
counterpart of a signature page of this Agreement.

13.6.  Headings.  The  headings  of the  Sections  hereof  are
inserted as a matter of convenience and for reference only and in no way define,
limit or describe the scope of this  Agreement  or the meaning of any  provision
hereof.

13.7.  Severability.  In the event that any  provision of this
Agreement or the application of any provision  hereof is declared to be illegal,
invalid or otherwise  unenforceable  by a court of competent  jurisdiction,  the
remainder of this Agreement shall not be affected except to the extent necessary
to delete such illegal,  invalid or unenforceable provision unless the provision
held invalid shall substantially  impair the benefit of the remaining portion of
this Agreement.

13.8.    Exculpation   Among   Purchasers.    Each   Purchaser
acknowledges and agrees that it is not relying upon any other Purchaser,  or any
officer, director, employee partner or affiliate of any such other Purchaser, in
making its investment or decision to invest in the Company or in monitoring such
investment.  Each Purchaser agrees that no Purchaser nor any controlling person,
officer,  director,  stockholder,  partner,  agent or employee of any  Purchaser
shall be liable for any action  heretofore  or hereafter  taken or omitted to be
taken  by any of them  relating  to or in  connection  with the  Company  or the
securities, or both.

13.9. Actions by Purchasers. Any actions permitted to be taken
by the Secured  Party and any  consents  required  to be obtained  from the same
under this Agreement, may be taken or given only by the Secured Party and if the
Secured  Party  takes any action or grants any  consent,  such action or consent
shall be deemed given or taken by all holders or Purchasers'  who

20

shall be bound by the decision or action taken by the Secured  Party without any
liability  on the part of the  Secured  Party to any  holder  or  Purchasers  of
securities hereunder.

13.10. Additional Purchasers.  Upon the execution and delivery
by any Person of this Agreement  after the date hereof with the written  consent
of the Company,  such Person shall be referred to as an Additional Purchaser and
shall become a Purchaser,  and each  reference in this  Agreement to "Purchaser"
shall also mean and be a reference to such Additional Purchaser.

14.      Confidential Investor Questionnaire.

14.1.  Each Purchaser  represents and warrants that he, she or
it comes within one of category A through H below.  ALL  INFORMATION IN RESPONSE
TO THIS SECTION WILL BE KEPT STRICTLY  CONFIDENTIAL.  The undersigned  agrees to
furnish any additional information which the Company deems necessary in order to
verify the answers set forth below.

Category A:              The  undersigned  is an individual  (not a partnership,
corporation, etc.) whose individual net worth, or joint
net worth  with his or her  spouse,  presently  exceeds
$1,000,000.

Explanation.  In  calculating  net worth you
may include equity in personal  property and
real  estate,   including   your   principal
residence,   cash,  short-term  investments,
stock and  securities.  Equity  in  personal
property and real estate  should be based on
the fair market value of such  property less
debt secured by such property.

Category B:              The  undersigned  is an individual  (not a partnership,
corporation,  etc.)  who had an  individual  income  in
excess  of  $200,000  in each of the  two  most  recent
years, or joint income with his or her spouse in excess
of  $300,000  in each of  those  years  (in  each  case
including  foreign  income,  tax exempt income and full
amount of capital  gains and losses but  excluding  any
income  of  other  family  members  and any  unrealized
capital appreciation) and has a reasonable  expectation
of reaching the same income level in the current year.

Category C:              The  undersigned is a director or executive  officer of
the Company which is issuing and selling the Notes.

Category D:              The   undersigned   is  a  bank;  a  savings  and  loan
association;  insurance company;  registered investment
company;   registered  business   development  company;
licensed small business investment company ("SBIC"); or
employee  benefit plan within the meaning of Title 1 of
ERISA and (a) the investment decision is made by a plan
fiduciary  which  is  either a bank,  savings  and loan
association, insurance company or registered investment
advisor,  or (b) the plan has total assets in excess of
$5,000,000 or is a self  directed plan with  investment
decisions  made solely by persons  that are  accredited
investors.

21

Category                 E: The  undersigned is a private  business  development
company  as  defined  in  section   202(a)(22)  of  the
Investment Advisors Act of 1940.

Category                 F:   The   undersigned   is   either   a   corporation,
partnership,    Massachusetts    business   trust,   or
non-profit  organization  within the meaning of Section
501(c)(3) of the Internal  Revenue  Code,  in each case
not formed for the specific  purpose of  acquiring  the
Notes and with total assets in excess of $5,000,000.

Category                 G: The  undersigned  is a trust  with  total  assets in
excess  of  $5,000,000,  not  formed  for the  specific
purpose of acquiring  the Notes,  where the purchase is
directed  by a  "sophisticated  person"  as  defined in
Regulation 506(b)(2)(ii) under the Securities Act.

Category                 H: The  undersigned  is an entity  (other than a trust)
all  the  equity   owners  of  which  are   "accredited
investors"  within one or more of the above categories.
If relying upon this Category alone,  each equity owner
must complete a separate copy of this Agreement.

The undersigned  agrees that the undersigned will notify the Company at any time
on or prior to the Final Closing Date in the event that the  representations and
warranties in this Agreement shall cease to be true, accurate and complete.

14.2.    Manner in Which Title Is To Be Held. (circle one)

(a)      Individual Ownership
(b)      Community Property
(c)      Joint Tenant with Right of
Survivorship (both parties
must sign)
(d)      Partnership*
(e)      Tenants in Common
(f)      Company*
(g)      Trust*
(h)      Other

*If  Notes  are  being  subscribed  for  by  an  entity,  the  attached
Certificate of Signatory must also be completed.

14.3.    NASD Affiliation.

Are you affiliated or associated with an NASD member firm (please check one):

Yes _________              No __________

If Yes, please describe:

_______________________________________________

22

_______________________________________________

_______________________________________________

*If Purchaser is a Registered  Representative with an NASD member firm, have the
following acknowledgment signed by the appropriate party:

The undersigned NASD member firm acknowledges  receipt of the notice required by
Article 3, Sections 28(a) and (b) of the Rules of Fair Practice.

__________________________________
Name of NASD Member Firm

By: ______________________________
Authorized Officer

Date: ____________________________

14.4.  Reliance on Confidential  Investor  Questionnaire.  The
undersigned  is informed  of the  significance  to the Company of the  foregoing
representations and answers contained in the Confidential Investor Questionnaire
contained  in this  Article 14 and such  answers  have been  provided  under the
assumption that the Company will rely on them.

23

IN WITNESS  WHEREOF,  the parties  hereto have  executed  this
Agreement as of the day and year indicated.

[Signature Page for each Purchaser]

$_____________  principal  amount of Notes,  at face value,  for an aggregate of
$___________ (the "Purchase Price")

By its execution and delivery of this signature page, the undersigned  Purchaser
hereby  joins in and  agrees  to be bound by the  terms  and  conditions  of the
Subscription  Agreement (the "Purchase  Agreement") by and among Hybridon,  Inc.
(the  "Company") and the Purchasers  (as defined  therein),  as to the aggregate
principal  amount of Notes set forth above and authorizes this signature page to
be attached to the Purchase Agreement or counterparts thereof.

--------------------------------         ---------------------------------
Signature                                Signature (if purchasing jointly)

--------------------------------         ---------------------------------
Name Typed or Printed                    Name Typed or Printed

--------------------------------         --------------------------------- 
Entity Name                              Entity Name                       

--------------------------------         --------------------------------- 
Address                                  Address                           

--------------------------------         --------------------------------- 
City, State and Zip Code                 City, State and Zip Code          

--------------------------------         --------------------------------- 
Telephone-Business                       Telephone--Business               

--------------------------------         --------------------------------- 
Telephone-Residence                      Telephone--Residence              

--------------------------------         --------------------------------- 
Facsimile-Business                       Facsimile--Business               

--------------------------------         --------------------------------- 
Facsimile-Residence                      Facsimile--Residence              

--------------------------------         --------------------------------- 
Tax ID # or Social Security #            Tax ID # or Social Security #     

Dated: ___________ _____, ___

This   Agreement   is   agreed   to   and   accepted   as   of
___________________, ____.

HYBRIDON, INC.

By:__________________________
Name:
Title:

24

CERTIFICATE OF SIGNATORY

(To be completed if Notes are
being subscribed for by an entity)

I,   ___________________,   am   the   ________________   (the
"Entity").  I certify that I am empowered  and duly  authorized by the Entity to
execute and carry out the terms of the Subscription Agreement, dated as of , and
to  purchase  and hold the Notes,  and  certify  further  that the  Subscription
Agreement  has been  duly and  validly  executed  on behalf  of the  Entity  and
constitutes a legal and binding obligation of the Entity.

IN  WITNESS  WHEREOF,  I have  set my hand  this  ____  day of
_________________, ____.

______________________________
(Signature)

25

EXHIBIT 23.1

CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS

As independent public accountants, we hereby consent to the use of our
report (and to all references to our Firm) included in or made a part of this
Registration Statement.

/s/ ARTHUR ANDERSEN LLP
Boston, Massachusetts
February 15, 2000