NEITHER THESE SECURITIES NOR THE COMMON STOCK ISSUABLE UPON EXERCISE HEREOF HAVE
BEEN  REGISTERED  UNDER  THE  SECURITIES  ACT OF  1933 OR ANY  APPLICABLE  STATE
SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED
OR OTHERWISE  TRANSFERRED IN THE ABSENCE OF A  REGISTRATION  STATEMENT IN EFFECT
WITH RESPECT TO THE  SECURITIES  UNDER SUCH ACT OR AN EXEMPTION  THEREFROM.  ANY
SUCH TRANSFER MAY ALSO BE SUBJECT TO APPLICABLE STATE SECURITIES LAWS.

                                 HYBRIDON, INC.


                  Class F Warrant for the Purchase of Shares of
                                  Common Stock


No. F-2                                              [2,750,000 in total] Shares


                  FOR VALUE  RECEIVED,  HYBRIDON,  INC., a Delaware  corporation
(the "Company"), hereby certifies that _______________ or its registered assigns
(the  "Holder")  is  entitled  to  purchase  from the  Company,  subject  to the
provisions of this Warrant (the "Warrant"), at any time on or after December 31,
2000 (the "Initial Exercise Date"),  and prior to 5:00 P.M., New York City time,
on  December  31,  2002 (the  "Termination  Date"),  [2,750,000]  fully paid and
non-assessable  shares of the  Common  Stock,  $.001 par value,  of the  Company
("Common Stock"), at an exercise price of $0.60 per share of Common Stock for an
aggregate  exercise  price of [one million six hundred  fifty  thousand  dollars
($1,650,000)]  (the  aggregate  purchase  price  payable for the Warrant  Shares
hereunder  is  hereinafter  sometimes  referred  to as the  "Aggregate  Exercise
Price").  The number of shares of Common Stock to be received  upon  exercise of
this Warrant and the price to be paid for each share of Common Stock are subject
to possible adjustment from time to time as hereinafter set forth. The shares of
Common Stock or other  securities or property  deliverable upon such exercise as
adjusted from time to time is hereinafter  sometimes referred to as the "Warrant
Shares." The exercise price of a share of Common Stock in effect at any time and
as adjusted from time to time is hereinafter  sometimes  referred to as the "Per
Share Exercise  Price." The Per Share Exercise Price is subject to adjustment as
hereinafter provided; in the event of any such adjustment, the number of Warrant
Shares shall also be adjusted,  by dividing the Aggregate  Exercise Price by the
Per Share  Exercise  Price in effect  immediately  after  such  adjustment.  The
Aggregate Exercise Price is not subject to adjustment.

                  1.       Exercise of Warrant.

                  (a) This Warrant may be exercised in whole or in part,  at any
time by its holder  commencing  on the  Initial  Exercise  Date and prior to the
Termination Date:

<PAGE>

                           (i) by  presentation  and  surrender of this Warrant,
                  together with the duly executed  subscription form attached at
                  the end hereof,  at the address set forth in  Subsection  8(a)
                  hereof,  together with payment,  by certified or official bank
                  check or wire transfer payable to the order of the Company, of
                  the Aggregate Exercise Price or the proportionate part thereof
                  if exercised in part; or

                           (ii) by  presentation  and surrender of this Warrant,
                  together  with  the  duly  executed   cashless  exercise  form
                  attached  at the end  hereof (a  "Cashless  Exercise")  at the
                  address set forth in Subsection  8(a) hereof.  The exchange of
                  Common  Stock for the  Warrant  shall  take  place on the date
                  specified in the Cashless Exercise Form or, if later, the date
                  the Cashless  Exercise Form is surrendered to the Company (the
                  "Exchange  Date").  Such  presentation  and surrender shall be
                  deemed  a  waiver  of  the  Holder's  obligation  to  pay  the
                  Aggregate Exercise Price, or the proportionate part thereof if
                  this Warrant is exercised in part.  In the event of a Cashless
                  Exercise,  this Warrant shall represent the right to subscribe
                  for and to  acquire  the  number of  shares  of  Common  Stock
                  (rounded to the next highest  integer) equal to (x) the number
                  of shares  of  Common  Stock  specified  by the  Holder in its
                  Cashless  Exercise Form (the "Total  Number") (such number not
                  to exceed the maximum number of shares of Common Stock subject
                  to this  Warrant,  as may be adjusted  from time to time) less
                  (y) the number of shares of Common Stock equal to the quotient
                  obtained by dividing  (A) the product of the Total  Number and
                  the  existing  Per  Share  Exercise  Price by (B) the  Current
                  Market Price (as defined in Subsection 3(h)).

                  (b) If this  Warrant is  exercised  in part only,  the Company
shall, upon presentation of this Warrant upon such exercise, execute and deliver
(along with the  certificate  for the Warrant  Shares  purchased)  a new Warrant
evidencing  the  rights of the  Holder  hereof to  purchase  the  balance of the
Warrant  Shares  purchasable  hereunder  upon the same terms and  conditions  as
herein set forth.  Upon proper  exercise of this Warrant,  the Company  promptly
shall deliver certificates for the Warrant Shares to the Holder duly legended as
authorized by the subscription  form. No fractional shares or scrip representing
fractional  shares shall be issued upon exercise of this Warrant;  provided that
the  Company  shall  pay to the  holder  of the  Warrant  cash  in  lieu of such
fractional shares.

                  2.  Reservation of Warrant Shares;  Fully Paid Shares;  Taxes.
The Company hereby  represents that it has, and until expiration of this Warrant
agrees that it shall,  reserve for  issuance or delivery  upon  exercise of this
Warrant,  such  number of shares of the Common  Stock as shall be  required  for
issuance  and/or delivery upon exercise of this Warrant in full, and agrees that
all Warrant Shares so issued and/or delivered will be validly issued, fully paid
and non-assessable,  and further agrees to pay all taxes and charges that may be
imposed upon such issuance and/or delivery.

                  3.       Protection Against Dilution.

                  (a) In case the Company shall  hereafter (i) pay a dividend or
make a  distribution  on its  Common  Stock in  shares  of  Common  Stock,  (ii)
subdivide its outstanding shares of Common Stock into a greater number of shares
or (iii) combine its outstanding shares


                                       2
<PAGE>

of Common Stock into a smaller  number of shares  (each of (i) through  (iii) an
"Action"),  the Per Share  Exercise  Price  shall be  adjusted  to be equal to a
fraction,  the numerator of which shall be the Aggregate  Exercise Price and the
denominator  of which  shall be the  number of  shares of Common  Stock or other
capital stock of the Company that the Holder would have held (solely as a result
of the  exercise of this Warrant and the  operation of such Action)  immediately
following  such Action if this Warrant had been exercised  immediately  prior to
such Action.  An adjustment  made pursuant to this  Subsection 3(a) shall become
effective  immediately  after  the  record  date in the  case of a  dividend  or
distribution and shall become effective  immediately after the effective date in
the case of a subdivision, combination or reclassification.

                  (b)  In  the   event   of  any   capital   reorganization   or
reclassification, or any consolidation or merger to which the Company is a party
other than a merger or  consolidation  in which the  Company  is the  continuing
corporation,  or in case of any sale or  conveyance  to  another  entity  of the
property of the Company as an entirety or  substantially  as an entirety,  or in
the case of any  statutory  exchange  of  securities  with  another  corporation
(including  any  exchange  effected  in  connection  with a  merger  of a  third
corporation  into the Company),  the Holder of this Warrant shall have the right
thereafter  to receive on the  exercise  of this  Warrant the kind and amount of
securities,  cash or other  property  which the Holder  would have owned or have
been   entitled   to   receive    immediately    after   such    reorganization,
reclassification,  consolidation, merger, statutory exchange, sale or conveyance
had this Warrant been exercised  immediately prior to the effective date of such
reorganization,  reclassification,  consolidation,  merger,  statutory exchange,
sale or conveyance  and in any such case, if necessary,  appropriate  adjustment
shall be made in the  application  of the provisions set forth in this Section 3
with  respect  to the  rights  and  interests  thereafter  of the Holder of this
Warrant  to the end that  the  provisions  set  forth  in this  Section  3 shall
thereafter  correspondingly be made applicable,  as nearly as may reasonably be,
in relation to any shares of stock or other  securities  or property  thereafter
deliverable  on the  exercise  of this  Warrant.  The above  provisions  of this
Subsection   3(b)  shall   similarly   apply  to   successive   reorganizations,
reclassifications,   consolidations,  mergers,  statutory  exchanges,  sales  or
conveyances.  The issuer of any shares of stock or other  securities or property
thereafter  deliverable on the exercise of this Warrant shall be responsible for
all of the agreements and obligations of the Company hereunder. A sale of all or
substantially  all of the assets of the Company for a  consideration  consisting
primarily  of  securities  shall be deemed a  consolidation  or  merger  for the
foregoing purposes.

                  (c)  Whenever  the  Per  Share  Exercise  Price  payable  upon
exercise of each  Warrant is adjusted  pursuant to this Section 3, the number of
shares of Common Stock underlying a Warrant shall  simultaneously be adjusted to
equal the number  obtained  by  dividing  the  Aggregate  Exercise  Price by the
adjusted Per Share Exercise Price.

                  (d) No  adjustment  in the Per Share  Exercise  Price shall be
required  unless  such  adjustment  would  require an increase or decrease of at
least $0.05 per share of Common Stock;  provided,  however, that any adjustments
which by reason of this  Subsection  3(d) are not  required  to be made shall be
carried  forward  and taken  into  account  in any  subsequent  adjustment.  All
calculations  under this  Section 3 shall be made to the nearest  cent or to the
nearest  1/100th of a share,  as the case may be.  Anything in this Section 3 to
the  contrary  notwithstanding,  the  Company  shall be  entitled  to make  such
reductions in the Per Share  Exercise  Price,  in addition to those  required by
this Section 3, as it in its discretion shall deem to

                                       3
<PAGE>

be  advisable  in order  that any  stock  dividend,  subdivision  of  shares  or
distribution   of  rights  to  purchase  stock  or  securities   convertible  or
exchangeable for stock hereafter made by the Company to its  stockholders  shall
not be taxable.

                  (e)  Whenever  the Per Share  Exercise  Price is  adjusted  as
provided in this Section 3 and upon any  modification  of the rights of a Holder
of Warrants in accordance with this Section 3, the Chief Financial  Officer,  or
equivalent  officer, of the Company shall promptly prepare a certificate setting
forth the Per Share  Exercise  Price and the number of Warrant Shares after such
adjustment or the effect of such  modification,  a brief  statement of the facts
requiring such adjustment or  modification  and the manner of computing the same
and cause copies of such certificate to be mailed to the Holder.

                  (f) If the Board of Directors of the Company shall declare any
dividend or other  distribution  with respect to the Common  Stock,  the Company
shall  mail  notice  thereof  to the  Holder no fewer  than 30 days prior to the
record date fixed for determining  stockholders  entitled to participate in such
dividend or other distribution.

                  (g) If, as a result of an  adjustment  made  pursuant  to this
Section 3, the Holder of any Warrant  thereafter  surrendered for exercise shall
become  entitled to receive  shares of two or more  classes of capital  stock or
shares of Common  Stock and other  capital  stock of the  Company,  the Board of
Directors (whose  determination  shall be conclusive and shall be described in a
written  notice to the Holder of any  Warrant  promptly  after such  adjustment)
shall  determine the allocation of the adjusted Per Share Exercise Price between
or among shares or such  classes of capital  stock or shares of Common Stock and
other capital stock.

                  (h) For the purpose of any computation  under Section 3 above,
the then Current  Market Price per share (the "Current  Market  Price") shall be
deemed to be the last sale price of the Common Stock on the trading day prior to
such date or, in case no such reported sales take place on such day, the average
of the last  reported  bid and asked  prices of the Common Stock on such day, in
either case on the principal  national  securities  exchange on which the Common
Stock is admitted to trading or listed,  or if not listed or admitted to trading
on any such exchange,  the representative  closing bid price of the Common Stock
as reported by the National  Association of Securities  Dealers,  Inc. Automated
Quotations  System  ("NASDAQ"),  or other similar  organization  if NASDAQ is no
longer reporting such information, or if not so available, the fair market value
of the Common Stock as determined  by the  Company's  Board of Directors in good
faith.

                  4.  Limited  Transferability.  This  Warrant  may not be sold,
transferred,  assigned or  hypothecated  by the Holder except in compliance with
the provisions of the Act and the applicable  state  securities "blue sky" laws,
and is so  transferable  only upon the books of the Company which it shall cause
to be maintained for such purpose.  The Company may treat the registered  Holder
of this  Warrant as he or it appears on the  Company's  books at any time as the
Holder for all purposes. The Company shall permit any Holder of a Warrant or his
duly authorized  attorney,  upon written request during ordinary business hours,
to inspect  and copy or make  extracts  from its books  showing  the  registered
holders of Warrants. All Warrants issued

                                       4
<PAGE>

upon the transfer or  assignment  of this Warrant will be dated the same date as
this Warrant,  and all rights of the holder  thereof shall be identical to those
of the Holder.

                  5.  Loss,   etc.,   of  Warrant.   Upon  receipt  of  evidence
satisfactory  to the Company of the loss,  theft,  destruction  or mutilation of
this Warrant, and of indemnity reasonably  satisfactory to the Company, if lost,
stolen or destroyed,  and upon surrender and  cancellation  of this Warrant,  if
mutilated,  the Company shall execute and deliver to the Holder a new Warrant of
like date, tenor and denomination.

                  6.       Investment Intent.

                  (a) The Holder represents,  by accepting this Warrant, that it
understands  that this Warrant and any  securities  obtainable  upon exercise of
this Warrant have not been registered for sale under Federal or state securities
laws and are  being  offered  and  sold to the  Holder  pursuant  to one or more
exemptions  from the  registration  requirements  of such  securities  laws. The
Holder is an "accredited  investor" within the meaning of Regulation D under the
Securities  Act of 1933, as amended (the "Act").  In the absence of an effective
registration of such securities or an exemption therefrom,  any certificates for
such  securities  shall bear the legend set forth on the first page hereof.  The
Holder understands that it must bear the economic risk of its investment in this
Warrant and any  securities  obtainable  upon  exercise  of this  Warrant for an
indefinite  period of time,  as this Warrant and such  securities  have not been
registered  under Federal or state  securities laws and therefore cannot be sold
unless  subsequently  registered under such laws,  unless as exemption from such
registration is available.

                  (b) The Holder,  by his acceptance of its Warrant,  represents
to the Company that it is acquiring this Warrant and will acquire any securities
obtainable  upon exercise of this Warrant for its own account for investment and
not with a view to, or for sale in connection with, any distribution  thereof in
violation  of the  Act.  The  Holder  agrees  that  this  Warrant  and any  such
securities will not be sold or otherwise  transferred  unless (i) a registration
statement  with  respect to such  transfer  is  effective  under the Act and any
applicable  state securities laws or (ii) such sale or transfer is made pursuant
to one or more exemptions from the Act.

                  7.  Status of Holder.  This  Warrant  does not confer upon the
Holder any right to vote or to consent to or receive  notice as a stockholder of
the Company, as such, in respect of any matters whatsoever,  or any other rights
or liabilities as a stockholder, prior to the exercise hereof.

                  8.  Notices.  No  notice  or other  communication  under  this
Warrant shall be effective unless, but any notice or other  communication  shall
be  effective  and shall be deemed to have been given if, the same is in writing
and is mailed by first-class mail, postage prepaid, addressed to:

                  (a)  the   Company   at  155   Fortune   Boulevard,   Milford,
         Massachusetts,  01757  Attention:  E. Andrews  Grinstead,  III, or such
         other  address as the Company has  designated in writing to the Holder;
         or

                  (b) the  Holder  at  [___________________________________]  or
         such  other  address as the  Holder  has  designated  in writing to the
         Company.

                                       5
<PAGE>

                  9.  Headings.  The headings of this Warrant have been inserted
as a matter of convenience and shall not affect the construction hereof.

                  10.  Applicable  Law.  This  Warrant  shall be governed by and
construed  in  accordance  with  the law of the  Commonwealth  of  Massachusetts
without giving effect to principles of conflicts of law thereof.


                                       6
<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]


                                       7
<PAGE>


                                  SUBSCRIPTION

                  The undersigned, ____________________________, pursuant to the
provisions  of the  foregoing  Warrant,  hereby  elects to  exercise  the within
Warrant to the extent of purchasing _____________________ shares of Common Stock
of Hybridon,  Inc.  thereunder and hereby makes payment of  $_______________  by
certified or official bank check in payment of the exercise price therefor.

Dated:_______________                       Signature:__________________________


         Address:_______________________________



                                CASHLESS EXERCISE

                  The undersigned, ____________________________, pursuant to the
provisions  of the  foregoing  Warrant,  hereby  elects to  exchange  the within
Warrant for ______________ shares of Common Stock of Hybridon,  Inc. pursuant to
the cashless exercise provisions of the Warrant. The undersigned hereby confirms
the representations and warranties made by it in the Warrant.

Dated:_______________                       Signature:__________________________


         Address:_______________________________


                                       8
<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  __________
shares of the Common Stock, no par value per share, of Hybridon, Inc. covered by
the foregoing  Warrant,  and a proportionate part of said Warrant and the rights
evidenced    thereby,    and   does    irrevocably    constitute   and   appoint
__________________________,  attorney,  to transfer that part of said Warrant on
the books of Hybridon, Inc.

Dated:_______________                       Signature:__________________________


         Address:_____________________________


                                       9