Document:

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:

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<PAGE>

         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.

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<PAGE>

         "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.

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<PAGE>

         "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.

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<PAGE>

         (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

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<PAGE>

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

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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.

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<PAGE>

         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

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<PAGE>

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
<PAGE>

                                    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
<PAGE>

         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
<PAGE>

         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
<PAGE>

         (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

<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

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