[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


<PAGE>
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