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                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

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

                                    FORM 8-K

                                 CURRENT REPORT

                     PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934


       Date of Report (Date of earliest event reported): January 13, 1998


                                     0-27352
                            (Commission File Number)

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

                                 HYBRIDON, INC.
             (Exact name of registrant as specified in its charter)

        Delaware                                            3072298
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  (State of Incorporation)                               (IRS Employer
                                                      Identification Number)

               620 Memorial Drive, Cambridge, Massachusetts 02139
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              (Address of registrant's principal executive office)

                                 (617) 528-7000
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                         (Registrant's telephone number)

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


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ITEM 5. Other Events


         Hybridon, Inc. (the "Company") and State Street Bank and Trust Company,
as trustee (the "Trustee"),  have entered into a First  Supplemental  Indenture,
dated as of January 13, 1998 (the  "Supplemental  Indenture").  The Supplemental
Indenture amended the Indenture, dated as of March 26, 1997, between the Company
and  Trustee  (the  "Original  Indenture"),   which  governs  the  Company's  9%
Convertible   Subordinated   Notes  due  2004  (the   "Notes").   The  resulting
modifications to the Original  Indenture affect the rights of the holders of the
Notes and subsequent  transferees.  Prior to making any investment decision with
respect to the Notes, all offerors and offerees of the Notes are urged to review
carefully the Supplemental Indenture, the text of which is attached hereto as an
exhibit and is  incorporated  herein by  reference,  and to  ascertain  from the
Company or otherwise  whether such Notes are Consenting Notes (as defined in the
Supplemental Indenture).


ITEM 7.   Financial Statements, Pro Forma Financial Information and Exhibits

          (c) Exhibits.

Exhibit Number                                       Title
- --------------                                       -----

     99             First Supplemental Indenture,  dated as of January 13, 1998,
                    by and  between  Hybridon,  Inc.  and State  Street Bank and
                    Trust Company, as Trustee






                                   SIGNATURES


         Pursuant to the  requirements  of the Securities  Exchange Act of 1934,
the  registrant  has duly  caused  this report to be signed on its behalf by the
undersigned, thereunto duly authorized.


                                         HYBRIDON, INC.

                                         By:   /s/ E. ANDREWS GRINSTEAD, III
                                               -----------------------------
                                               Name:   E. Andrews Grinstead, III
                                               Title:  Chairman of the Board, 
                                                       President and Chief
                                                       Executive Officer

                                         Date:  January 15, 1998


                          FIRST SUPPLEMENTAL INDENTURE

                                     between

                                 HYBRIDON, INC.

                                       and

                      STATE STREET BANK AND TRUST COMPANY,

                                   as Trustee

                          Dated as of January 13, 1998








                                TABLE OF CONTENTS

(This Table of Contents is not a part of the First Supplemental Indenture and is
only for convenience of reference)

Section 1.        Definitions and Interpretations

Section 2.        Amendment of Section 1.1

Section 3.        Amendment of Section 2.1(c)

Section 4.        Amendment of Section 2.6

Section 5.        Amendment of Section 2.10(b)

Section 6.        Amendment of Section 4.1

Section 7.        Amendment of Article 10

Section 8.        Consent and Waiver

Section 9.        Severability

Section 10.       Counterparts

Section 11.       Applicable Provisions of Law

Section 12.       Captions

Section 13.       Original Indenture Affirmed

Section 14.       Effective Date

Signatures









                          FIRST SUPPLEMENTAL INDENTURE

         This FIRST  SUPPLEMENTAL  INDENTURE  dated as of January  13, 1998 (the
"First Supplemental Indenture"),  is between Hybridon, Inc. (the "Company"), and
State  Street Bank and Trust  Company,  as trustee (the  "Trustee").  This First
Supplemental Indenture supplements the Indenture dated as of March 26, 1997 (the
"Original  Indenture"),  between the Company and the Trustee, which provided for
the issuance of the Company's 9% Convertible  Subordinated  Notes Due 2004. This
First  Supplemental  Indenture  is entered  into  pursuant to Section 9.2 of the
Original  Indenture  for the  purpose  of (i)  supplementing  and  amending  the
definition  of "Senior  Indebtedness"  set forth in Section 1.1 of the  Original
Indenture and for the purpose of imposing  certain  additional  provisions  with
respect to the ownership of, and payment of interest on, the Notes, in each case
only as to the Consenting Notes (as defined herein), and (ii) for the purpose of
waiving  compliance by the Company with certain  covenants and  restrictions set
forth in the Original Indenture, all as provided herein.

         Whereas the Holders of a majority of the outstanding  principal  amount
of Notes have  authorized the execution and delivery of this First  Supplemental
Indenture, and in consideration of the mutual agreements contained in this First
Supplemental Indenture and other good and valuable consideration, the receipt of
which is hereby  acknowledged,  the Company  and the Trustee  agree as set forth
herein  for  their  own  benefit  and  for the  benefit  of the  Holders  of all
outstanding Notes under the Indenture:

         Section 1. Definitions and Interpretations.

         (a)  Whenever  used  in  the  Original   Indenture  or  in  this  First
Supplemental  Indenture,  "Indenture"  shall  mean the  Original  Indenture,  as
amended and supplemented by this First Supplemental Indenture.

         (b) Unless  otherwise  defined  or  amended in this First  Supplemental
Indenture,  or unless the context otherwise  requires,  the terms defined in the
Original  Indenture  shall have the same  meanings  in this  First  Supplemental
Indenture.

         Section 2. Amendment of Section 1.1

         (a) a new defined term "2007 Notes" is added to Section 1.1 as follows:

                  "2007 Notes" means: (i) up to $55,000,000  principal amount of
                  the Company's  Notes due 2007 (the "2007 Notes") to be sold to
                  purchasers in the unit offering (the "Offering")  contemplated
                  by a certain Unit Purchase Agreement in substantially the form
                  attached in Exhibit A hereto (the "Unit  Purchase  Agreement")
                  and pursuant to a substantially  similar agreement on the same
                  economic  terms (the "U.S.  Unit  Purchase  Agreement")  to be
                  entered into in connection with sales to be made to purchasers
                  who include U.S. persons, of units (the "Units") consisting of
                  up  to  $55,000,000   principal  amount  of  2007  Notes  (the
                  "Offering Notes") (or, under certain circumstances, the


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                  number  of  shares of Series B  Preferred  Stock  which  would
                  otherwise  underlie  such  Offering  Notes)  and  warrants  to
                  purchase  the number of shares of Common  Stock of the Company
                  set forth in Section 2.2 of the Unit  Purchase  Agreement,  at
                  the  exercise  price  stated  therein  ("Warrants")  (the Unit
                  Purchase  Agreement,  the U.S. Unit Purchase Agreement and the
                  other  documents  and  agreements  attached in Exhibit A being
                  collectively referred to herein as the "2007 Note Documents");
                  (ii) the 2007 Notes underlying  certain Placement and Advisory
                  Warrants  referred  to  in  the  Unit  Purchase  Agreement  to
                  purchase Units equal to 25% of the Units sold to purchasers in
                  the Offering (which will be purchased by the Placement  Agents
                  (as  defined  in  the  Unit   Purchase   Agreement)  or  their
                  designees),  exercisable for seven years commencing six months
                  after the final  closing  date of the  Offering at an exercise
                  price  equal  to 110% of the  initial  offering  price  of the
                  Units;  (iii) any 2007 Notes  issued as interest on 2007 Notes
                  in  accordance  with the 2007  Note  Documents;  and (iv) 2007
                  Notes  issuable  in the case of  lost,  stolen,  destroyed  or
                  mutilated 2007 Notes.

         (b) The definition of "Senior Indebtedness" set forth in Section 1.1 of
the Original  Indenture is hereby amended by adding the following final sentence
thereto:

                  Notwithstanding anything contained in the foregoing proviso or
                  elsewhere in this Indenture,  (A) the indebtedness represented
                  by Notes  beneficially  owned (at the time of their signature)
                  by each person that has executed a Consent and Waiver dated as
                  of January  12,  1998 (the  "Consent")  and that has, if not a
                  record holder of Notes, caused its DTC Participant (as defined
                  therein) to execute,  on such person's behalf, a substantially
                  similar  Consent and Waiver,  which persons (and the aggregate
                  principal amount of Notes  beneficially  owned by each of them
                  (in the case of beneficial  interests in a Global Security) or
                  owned by them of record,  in the case of definitive Notes) are
                  identified on the attached  Schedule A (each such person being
                  individually  referred to herein as a "Consent  Signatory" and
                  collectively,  as  the  "Consent  Signatories")  and  (B)  the
                  indebtedness  represented  by Notes  held by every  subsequent
                  Holder  of a  Note  evidencing  the  same  debt  as a  Consent
                  Signatory's Note ((A) and (B),  collectively,  the "Consenting
                  Notes"), including the payment of the Principal and repurchase
                  price, if any, of and interest on, and all other amounts which
                  may become payable in respect of, such  Consenting  Notes from
                  time to time,  shall be  subordinated  and subject in right of
                  payment to the prior  payment in full of the  principal of and
                  interest  on and all other  amounts  which may become due with
                  respect to the 2007 Notes on the terms and conditions provided
                  in  Article 11 of the  Indenture  (and not pari passu with the
                  2007  Notes) and the 2007  Notes  will be,  for all  purposes,
                  "Senior  Indebtedness"  as defined herein with respect to such
                  Consenting Notes.

         Section 3. Amendment of Section 2.1(c).


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         Section 2.1(c) of the Original Indenture is hereby amended by adding at
the end of the first paragraph thereof,  "Notwithstanding the foregoing,  owners
of  beneficial  interests  in Global  Securities  will be  entitled  to  receive
physical delivery of certificated Notes as provided in Section 2.6(d)(i)(D)."

         Section 4. Amendment of Section 2.6.

         (a) Section  2.6(d)(i) of the Original  Indenture is hereby  amended by
inserting a reference  to clause  ("D") on the third line  thereof and by adding
thereto the following new subsection ("D"):

          (D)  in connection  with the issuance by the Company of its 2007 Notes
               to which the indebtedness  represented by the Consenting Notes is
               subordinated,  the  Consent  referred  to in  Section  1.1 of the
               Indenture, whereupon definitive Notes (or beneficial interests in
               one or more new Global  Securities  provided  for by the  Company
               pursuant to Section  2.1(a) to represent such  Consenting  Notes)
               shall be  issued as  hereinafter  provided,  and such  Consenting
               Notes shall be Transfer Restricted  Securities until such time as
               the  Company  shall  have  caused  any   effective   registration
               statement covering the resale of the Notes to reflect the matters
               contemplated  herein  (whereupon  the Company shall so advise the
               Trustee  in  writing  that the  Consenting  Notes  are no  longer
               Transfer  Restricted  Securities),  and until the  Trustee  is so
               advised, the Certificates representing any Consenting Notes shall
               include  the  following  additional  legend as to their  modified
               subordination, transfer restriction and payment provisions:

         AS PROVIDED IN A FIRST SUPPLEMENTAL INDENTURE AS TO WHICH THE HOLDER OF
         THIS NOTE (OR A PREDECESSOR  HOLDER  THEREOF) HAS CONSENTED,  A COPY OF
         WHICH WILL BE  FURNISHED BY THE COMPANY  UPON  REQUEST,  (i) THE SENIOR
         INDEBTEDNESS  TO WHICH  THE NOTE  REPRESENTED  BY THIS  CERTIFICATE  IS
         SUBORDINATED  INCLUDES THE COMPANY'S  NOTES DUE 2007, (ii) THE TRANSFER
         OF  THIS  NOTE  IS  FURTHER   RESTRICTED  AS  PROVIDED  IN  SUCH  FIRST
         SUPPLEMENTAL  INDENTURE,  AND (iii) THE  REQUIRED  PAYMENT OF  INTEREST
         OTHERWISE  DUE ON APRIL 1, 1998 WITH RESPECT TO THE NOTES  EVIDENCED BY
         THIS  CERTIFICATE  HAS BEEN DEFERRED TO OCTOBER 1, 1998 IF THIS NOTE IS
         OUTSTANDING ON SUCH LATER DATE.


         (b) Section  2.6(i) is hereby  amended by adding  thereto the following
subsection (vi):

                  (vi)     in connection with the issuance by the Company of its
                           2007  Notes  to  which  the   Consenting   Notes  are
                           subordinated,  any Notes of a Consenting  Holder that
                           are  in  definitive   form  shall  be  exchanged  for
                           Consenting  Notes in definitive  form as described in
                           subsection 2.6(d)(i)(D).


                                       -3-




         Section  5.  Amendment  of  Section  2.10(b).  Section  2.10(b)  of the
Original Indenture is hereby amended by adding at the end thereof,  "or (iii) is
pursuant to Section 2.6(d)(i)(D)."

         Section 6.  Amendment of Section 4.1.  Section 4.1 is hereby amended by
adding the following final paragraph thereto:

                  Notwithstanding anything to the contrary herein (including the
                  definition  of  Senior  Indebtedness)  or in  the  Notes,  the
                  required  payment of interest  that would be otherwise due and
                  payable on April 1, 1998, with respect to the Consenting Notes
                  shall be deferred to, and shall be payable on, October 1, 1998
                  (to the extent such  Consenting  Notes are outstanding on such
                  later date),  to the Holders of record  otherwise  entitled to
                  such required  payment of interest,  with the same effect (and
                  without any additional accrual of interest thereon) as if paid
                  on such April 1, 1998 Interest Payment Date.

         Section 7. Amendment of Article 10. There is hereby added the following
new Section 10.10:

         Section 10.10 Exchange for Series A Preferred Stock

               As described in the 2007 Note  Documents,  and in addition to the
          conversion  rights  contained in the Original  Indenture,  the Company
          contemplates  offering  Noteholders  the opportunity to exchange their
          outstanding  Notes for  shares of the  Company's  authorized  Series A
          Preferred  Stock issued  pursuant to a Certificate  of  Designation in
          substantially the form provided for in Exhibit B hereto (the "Series A
          Preferred  Stock") and certain other  securities;  provided,  however,
          that no such offer is made  hereby and any such  exchange  offer shall
          only be made by the Company's  actual  exchange offer  documents.  The
          Trustee  shall  have  no  responsibility  with  respect  to  any  such
          exchange,  either as an exchange agent or otherwise,  except as it may
          expressly so agree in a separate written  agreement between it and the
          Company.

         Section 8. Consent and Waiver.  Notwithstanding  any  provisions of the
Indenture to the contrary, the issuance by the Company of (i) the Offering Notes
until October 15, 1998,  (ii) any other 2007 Notes at any time,  (iii) shares of
Series B Preferred  Stock issued  pursuant to a Certificate  of  Designation  in
substantially  the form  included in Exhibit A (the "Series B Preferred  Stock")
and (iv) shares of Series A Preferred Stock (including  Series A Preferred Stock
issued  as  dividends  thereon),  and  the  performance  by the  Company  of its
obligations thereunder and under the 2007 Note Documents, shall not be deemed to
constitute  a breach of any  covenant  or  agreement  set forth in the  Original
Indenture,  as such  breach has been  waived by the  Holders  of a  majority  in
aggregate  principal  amount  of  the  Notes  pursuant  to  Section  9.2  of the
Indenture.  Notwithstanding  any  provisions  of the  Indenture  to the contrary
(including,  without  limitation,   Sections  1.1,  4.9,  4.10  and  the  notice
provisions of Section 10.9(a)), the transactions contemplated by the 2007


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Note Documents (including,  without limitation,  the compensation and securities
payable to an  affiliate  of the  Company  identified  in the Consent and Waiver
referred to above (the "Affiliate"),  as placement agent, in connection with the
issuance of the  Company's  2007 Notes,  and the right of the  Affiliate  or its
affiliates to purchase Units, 2007 Notes, Series B Preferred Stock, Warrants and
Placement and Advisory  Warrants)  shall not be deemed to constitute a breach of
any  covenant or  agreement  set forth in the  Original  Indenture,  as any such
breach has been waived by Holders of a majority in aggregate principal amount of
the Notes.

         Section 9.  Severability.  If any provision of this First  Supplemental
Indenture  shall  be held  or  deemed  to be or  shall,  in  fact,  be  illegal,
inoperative or  unenforceable,  the same shall not affect any other provision or
provisions  herein  contained  or  render  the  same  invalid,   inoperative  or
unenforceable to any extent whatever.

         Section 10.  Counterparts.  This First  Supplemental  Indenture  may be
executed in several counterparts,  each of which shall be an original and all of
such shall, taken together, constitute but one and the same instrument.

         Section  11.  Applicable  Provisions  of Law.  This First  Supplemental
Indenture  shall be governed by and construed in accordance with the laws of the
State of New York without regard to the conflict of law provisions thereof.

         Section  12.  Captions.   The  captions  and  headings  in  this  First
Supplemental  Indenture are for convenience  only and in no way define,  expand,
limit or  describe  the scope or intent of any  provisions  or  sections of this
First Supplemental Indenture.

         Section 13. Original Indenture Affirmed.  Except as otherwise expressly
supplemented or amended by this First Supplemental Indenture,  the provisions of
the Original  Indenture shall remain unchanged,  binding,  and in full force and
effect.

         Section 14.  Effective Date. The provisions of this First  Supplemental
Indenture  shall take effect on the date of execution and delivery of this First
Supplemental Indenture by the parties hereto.


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         IN WITNESS  WHEREOF,  the  Company  has  caused  these  presents  to be
executed  in its name by its  duly  authorized  officer  and its  official  seal
hereunto  affixed;  and to evidence its acceptance of the trusts hereby created,
the Trustee has caused these  presents to be executed in its corporate  name and
with its corporate seal hereunto affixed,  by its duly authorized officer, as of
the date first above written.

[SEAL]                                    HYBRIDON, INC.



                                          By:  /s/ E. ANDREWS GRINSTEAD, III
                                             -------------------------------
                                          Name:  E. Andrews Grinstead, III
                                          Title:    Chairman and President

[SEAL]                                    STATE STREET BANK AND TRUST
                                          COMPANY, as Trustee



                                          By: /s/ Earl W. Dennison, Jr.
                                             -------------------------------
                                          Name:  Earl W. Dennison, Jr.
                                          Title:    Vice President


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