Document:

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

<PAGE>

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

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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

<PAGE>

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

<PAGE>

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;

<PAGE>

                  (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

<PAGE>

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

<PAGE>

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.

<PAGE>

                  (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.

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Essam A. J. Alamdar                                  Riyadh-11431
                                                     Saudi Arabia

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Mansour S. M. A. Al-Sharif

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

H.K. Properties Limited

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                    Address of Holder

Kincroft Limited                                  Ridgeway House, Ridgeway St.
                                                  Douglas, Isle of Man

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Fouad M. O. Tawfig

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Fouad M. O. Tawfig

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Fouad M. O. Tawfig

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Seif Foundation

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Bajrai Int'l Group Ltd.

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Darier, Hentsch & Cie

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Solter Corporation

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Malek Salam and Oussama Salam

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Mohamad Khaled Omari

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Khaled M. K. Abdulghani

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Sylvigne Abu Haidar

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Nicris Limited

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Clapham Investments Limited

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Torben Duer

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Nafez M. M. Al-Jindi

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Alain Mallart

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Imad Moustapha Mansour

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Oussama M. Salam

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

LGT Bank in Liechtenstein AG

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Investeringsselskabet af 1/12 1997 Ap.S

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

MicroTech Software A/S

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

JSP Holding Ap.S

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Motasim F. Hajaj

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Mohamad A. Bajria

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

E. Andrews Grinstead, III

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Essam A. J. Alamdar

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Pillar Investments Ltd.                              28 Avenue de Messine
                                                     Paris, France 75008

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Christopher S. Gaffney                               3 Winthrop Street
                                                     West Newton, MA  02465

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Jonathan P. Raymond                                  133 Park St., #1207
                                                     Brookline, MA  02446

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Forum Capital Markets LLC                            53 Forest Ave.
                                                     Old Greenwich, CT  06870

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

Samuel Morrill Robbins                               300 Prince Street
                                                     West Newton, MA  02465

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of Holder

<PAGE>

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

<PAGE>

         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:

<PAGE>

                                   SCHEDULE A

Name of Holder                                       Address of HolderTHE 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
<PAGE>

                  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
<PAGE>

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
<PAGE>

                  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

<PAGE>

                  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

<PAGE>

                                  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

<PAGE>

                                   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

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