<PAGE>
                                                                    Exhibit 10.8

                                 HYBRIDON, INC.

                          REGISTRATION RIGHTS AGREEMENT

                            dated as of May 20, 2005
<PAGE>
                                 HYBRIDON, INC.

                          REGISTRATION RIGHTS AGREEMENT

     This Registration Rights Agreement (this "AGREEMENT") is entered into as of
May 20, 2005 by and among Hybridon, Inc., a Delaware corporation (the
"COMPANY"), the persons and entities listed on the Schedule of Purchasers
attached hereto as Exhibit A (the "PURCHASERS") and the entity listed on the
Agent Schedule attached hereto as Exhibit B (the "AGENT"). The Purchasers and
the Agent shall become parties to this Agreement by the execution and delivery
of counterpart signature pages hereto in a form reasonably satisfactory to the
Company.

     WHEREAS, the Company is conducting an offering (the "OFFERING") of
convertible subordinated notes (the "NOTES"), as described in the Confidential
Private Placement Memorandum dated April 26, 2005 (the "MEMORANDUM");

     WHEREAS, either the Company or the Purchasers may, under specified
circumstances, cause the Notes to be converted into shares of the Company's
common stock, $0.001 par value per share (the "COMMON STOCK"), as described in
the Memorandum;

     WHEREAS, the Company may pay accrued interest on the Notes by issuing to
the Purchasers shares of Common Stock under the terms of the Notes (the
"INTEREST SHARES");

     WHEREAS, in connection with the Offering, the Company has engaged the Agent
and has agreed to issue to the Agent a warrant (the "AGENT WARRANT") to purchase
shares of Common Stock; and

     WHEREAS, to induce the Purchasers to purchase Notes in the Offering and the
Agent to assist the Company in the Offering, the Company has agreed to provide
certain registration rights under the Securities Act (as defined below) and
applicable state securities laws;

     NOW, THEREFORE, in consideration of the promises and the mutual covenants
contained herein and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Company, the Agent and each of
the Purchasers hereby agree as follows:

     1. Certain Definitions. As used in this Agreement, the following terms
shall have the following meanings:

          (a) "Business Day" means any day other than Saturday, Sunday or any
other day on which commercial banks in The City of New York or in Boston,
Massachusetts are required by law to remain closed.

          (b) "Commission" means the Securities and Exchange Commission, or any
other federal agency at the time administering the Securities Act.

          (c) "Exchange Act" means the Securities Exchange Act of 1934, as
amended, or any successor federal statute, and the rules and regulations of the
Commission issued under such Act, as they each may, from time to time, be in
effect.


                                       1
<PAGE>
          (d) "Indemnified Party" means a party entitled to indemnification
pursuant to Section 7.

          (e) "Indemnifying Party" means a party obligated to provide
indemnification pursuant to Section 7.

          (f) "Person" means an individual, a limited liability company, a
partnership, a joint venture, a corporation, a trust, an unincorporated
organization or association and governmental or any department or agency
thereof.

          (g) "Registrable Securities" means (i) the shares of Common Stock
issuable upon conversion of the Notes, (ii) the shares of Common Stock issued or
issuable upon exercise of the Agent Warrant, (iii) solely in the context of an
Interest Payment Registration Statement, the Interest Shares, and (iv) any other
shares of Common Stock issued in respect of such shares (as a result of a stock
split, stock dividend, reclassification, recapitalization or other similar
transaction affecting the Common Stock); provided, however, that shares of
Common Stock that are Registrable Securities shall cease to be Registrable
Securities upon the earliest of (A) the date that such shares are eligible to be
sold under Rule 144 of the Securities Act, without restriction by the volume
limitations of Rule 144(e) of the Securities Act, (B) the date that such shares
are sold (I) pursuant to a registration statement, (II) to or through a broker,
dealer or underwriter in a public securities transaction and/or (III) in a
transaction exempt from the registration and prospectus delivery requirements of
the Securities Act such that all transfer restrictions and restrictive legends
with respect thereto, if any, are removed upon the consummation of such sale, or
(C) any sale or transfer to any Person which by virtue of Section 9 of this
Agreement is not entitled to the rights provided by this Agreement. Wherever
reference is made in this Agreement to a request or consent of holders of a
certain percentage of Registrable Securities, the determination of such
percentage shall include shares of Common Stock issuable upon exercise of the
Agent Warrant even if such exercise has not been effected.

          (h) "Registration Statement" means a registration statement of the
Company filed under the Securities Act and covering the Registrable Securities.

          (i) "Rightsholders" means the Purchasers, the Agent and any persons or
entities to whom the rights granted under this Agreement are transferred by any
Purchaser, Agent or his or its successors or assigns pursuant to Section 9 of
this Agreement.

          (j) "Securities Act" means the Securities Act of 1933, as amended, or
any successor federal statute, and the rules and regulations of the Commission
issued under such Act, as they each may, from time to time, be in effect.

     2. Registration

          (a) The Company shall use its best efforts to prepare and file with
the Commission a Registration Statement (the "Primary Registration Statement")
covering the resale of all of the Registrable Securities and such other shares
of Common Stock as the Company may be required to include pursuant to
registration rights agreements with other Persons within 60 days of the date
hereof. The Company shall use its best efforts to have the Primary Registration


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<PAGE>
Statement declared effective by the Commission within 90 days after the date the
Primary Registration Statement is filed or as soon as possible thereafter.

          (b) The Company shall be permitted hereunder to amend the Registration
Statement filed by the Company pursuant to Section 2(a) to cover the resale of
any Interest Shares issued or to be issued by the Company, without the need to
obtain any consent of the Rightsholders, whether such amendment is filed as a
pre-effective or post-effective basis. In addition, the Company may file
hereunder one or more additional Registration Statements covering the resale of
Interest Shares issued or to be issued by the Company (an "Interest Payment
Registration Statement"). In the event that the Company files an Interest
Payment Registration Statement, the Company and the Rightsholders shall have all
of the rights and obligations with respect to such Interest Payment Registration
Statement and the Interest Shares that the Rightsholders have under this
Agreement with respect to the Primary Registration Statement and the Registrable
Securities.

          (c) The Company shall use its best efforts to cause each Registration
Statement to remain effective until the date on which the Rightsholders do not
hold any Registrable Securities covered by such Registration Statement.

     3. Registration Procedures.

          (a) In connection with the effectiveness of the Registration
Statement, the Company shall furnish to each Rightsholder such reasonable
numbers of copies of the prospectus and such documents incident thereto,
including any amendment of or supplement to the prospectus, as a Rightsholder
from time to time may reasonably request in order to facilitate the disposition
of such Rightsholder's Registrable Securities under the Registration Statement
in conformity with the requirements of the Securities Act.

          (b) The Company shall use its best efforts to register or qualify the
Registrable Securities covered by the Registration Statement under the
securities laws of such states of the United States as the Rightsholders may
reasonably request; provided, however, that the Company shall not be required in
connection with this paragraph (b) to qualify as a foreign corporation or
execute a general consent to service of process in any jurisdiction.

          (c) If the Company has delivered preliminary or final prospectuses to
the Rightsholders and if after having done so the Company determines that the
prospectus and/or the Registration Statement needs to be amended or supplemented
to comply with the requirements of the Securities Act, the Company shall
promptly notify the Rightsholders and, if requested by the Company, the
Rightsholders shall immediately cease making offers or sales of shares under the
Registration Statement and shall return all prospectuses to the Company. The
Company shall as promptly as reasonably practicable prepare and file with the
Commission any required amendment or supplement and following such filing, and,
if applicable, the effectiveness of such filing, shall provide the Rightsholders
with revised or supplemented prospectuses. Following receipt of the revised or
supplemented prospectuses, the Rightsholders shall be free to resume making
offers and sales under the Registration Statement.


                                       3
<PAGE>
          (d) The Company shall use its best efforts to cause all such
Registrable Securities registered pursuant to this Agreement to be listed on
each securities exchange on which similar securities issued by the Company are
then listed.

     4. Limitations on Registration Rights.

          (a) The Company may, by written notice to the Rightsholders, (i) delay
the filing of, or effectiveness of, a Registration Statement or (ii) suspend a
Registration Statement after effectiveness and require that the Rightsholders
immediately cease sales of shares pursuant to the Registration Statement, in the
event that the Company is engaged in any activity or transaction or preparations
or negotiations for any activity or transaction that the Company desires to keep
confidential for business reasons, if the Company determines in good faith that
the public disclosure requirements imposed on the Company under the Securities
Act in connection with the Registration Statement would require disclosure of
such activity, transaction, preparations or negotiations.

          (b) If the Company requires the Rightsholders to cease sales of shares
pursuant to paragraph (a) above, the Company shall, as promptly as practicable
following the termination of the circumstance which entitled the Company to do
so, take such actions as may be necessary to reinstate the effectiveness of the
Registration Statement and/or give written notice to all Rightsholders
authorizing them to resume sales pursuant to the Registration Statement. If as a
result thereof the prospectus included in the Registration Statement has been
amended to comply with the requirements of the Securities Act, the Company shall
enclose such revised prospectus with the notice to Rightsholders given pursuant
to this paragraph (b), and the Rightsholders shall make no offers or sales of
shares pursuant to the Registration Statement other than by means of such
revised prospectus.

          (c) Notwithstanding the foregoing, the Company may not (i) delay the
filing of, or the effectiveness of, a Registration Statement or (ii) suspend a
Registration Statement, pursuant to paragraph (a) above on more than two
occasions during any 12-month period or for more than 60 days per such occasion.

     5. Obligations of the Rightsholders.

          (a) The Company shall not be required to include any Registrable
Securities in a Registration Statement unless such Rightsholder shall have
furnished to the Company such information regarding itself, the Registrable
Securities held by it and the intended method of disposition of the Registrable
Securities held by it as shall be reasonably required by the Company to effect
the effectiveness of the Registration Statement and unless such Rightsholder
shall have executed such documents in connection with the Registration Statement
as the Company may reasonably request. Each Rightsholder shall promptly notify
the Company of any material change with respect to such information previously
provided to the Company by such Rightsholder, including without limitation
notice of the sale by the Rightsholder of any Registrable Securities.


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<PAGE>
          (b) Each Rightsholder agrees to cooperate with the Company as
reasonably requested by the Company in connection with the preparation and
filing of the Registration Statement hereunder.

     6. Expenses of Registration. The Company shall pay the expenses incurred by
it in complying with its obligations under this Agreement, including all
registration and filing fees, exchange listing fees, fees and expenses of
counsel for the Company, and fees and expenses of accountants for the Company,
but excluding (a) any brokerage fees, selling commissions or underwriting
discounts incurred by the Rightsholders in connection with sales under the
Registration Statement and (b) the fees and expenses of any counsel retained by
Rightsholders.

     7. Indemnification and Contribution.

          (a) In the event of any registration of any of the Registrable
Securities under the Securities Act pursuant to this Agreement, the Company will
indemnify and hold harmless each Rightsholder, each of its officers, directors
and partners, and each underwriter of such Registrable Securities, if any, and
each other person, if any, who controls such Rightsholder or underwriter within
the meaning of the Securities Act or the Exchange Act against any and all
losses, claims, damages or liabilities, joint or several, to which such
Rightsholder, underwriter or controlling person may become subject under the
Securities Act, the Exchange Act, state securities or Blue Sky laws or
otherwise, insofar as such losses, claims, damages or liabilities (or actions in
respect thereof) arise out of or are based upon (i) any untrue statement or
alleged untrue statement of any material fact contained in the Registration
Statement, registering such Registrable Securities, any preliminary prospectus
or final prospectus contained in such Registration Statement, or any amendment
or supplement to such Registration Statement or (ii) the omission or alleged
omission to state a material fact required to be stated therein or necessary to
make the statements therein not misleading; and the Company will reimburse such
Rightsholder, underwriter and each such controlling person for any legal or any
other expenses reasonably incurred by such Rightsholder, underwriter or
controlling person in connection with investigating or defending any such loss,
claim, damage, liability or action; provided, however, that the Company will not
be liable in any such case to the extent that any such loss, claim, damage,
liability or action arises out of or is based upon any untrue statement or
omission made in such Registration Statement, preliminary prospectus or final
prospectus, or any such amendment or supplement, in reliance upon and in
conformity with information furnished to the Company by or on behalf of such
Rightsholder, underwriter or controlling person and stated to be specifically
for use in connection with such Registration Statement; and provided further
that the foregoing indemnity agreement is subject to the condition that, insofar
as it relates to any untrue statement, alleged untrue statement, omission or
alleged omission made in any Registration Statement, prospectus or amendment or
supplement that was eliminated, remedied or cured by the Company, such indemnity
agreement shall not inure to the benefit of any Rightsholder from whom the
Person asserting any loss, claim, damage or liability purchased the Registrable
Securities if a copy of the Registration Statement, prospectus, amendment or
supplement was provided by the Company to the Rightsholder but was not given or
sent to such Person by the Rightsholder prior to written confirmation of such
sale.

          (b) Each Rightsholder, severally and not jointly, will indemnify and
hold harmless the Company, each of its directors and officers and each
underwriter (if any) and each


                                       5
<PAGE>
person, if any, who controls the Company or any such underwriter within the
meaning of the Securities Act or the Exchange Act, against any losses, claims,
damages or liabilities, joint or several, to which the Company, such directors
and officers, underwriter or controlling person may become subject under the
Securities Act, Exchange Act, state securities or Blue Sky laws or otherwise,
insofar as such losses, claims, damages or liabilities (or actions in respect
thereof) arise out of or are based upon (i) any untrue statement or alleged
untrue statement of a material fact contained in such Registration Statement,
any preliminary prospectus or final prospectus contained in such Registration
Statement, or any amendment or supplement to such Registration Statement, or
(ii) any omission or alleged omission to state a material fact required to be
stated therein or necessary to make the statements therein not misleading, if
and to the extent (and only to the extent) that the statement or omission was
made in reliance upon and in conformity with written information relating to
such Rightsholder furnished to the Company by such Rightsholder and stated to be
specifically for use in connection with such Registration Statement, prospectus,
amendment or supplement; provided, however, that the obligations of a
Rightsholder hereunder shall be limited to an amount equal to the net proceeds
to such Rightsholder of Registrable Securities sold in connection with such
registration.

          (c) Each Indemnified Party shall give notice to the Indemnifying Party
promptly after such Indemnified Party has actual knowledge of any claim as to
which indemnity may be sought, and shall permit the Indemnifying Party to assume
the defense of any such claim or any litigation resulting therefrom; provided,
that counsel for the Indemnifying Party, who shall conduct the defense of such
claim or litigation, shall be approved by the Indemnified Party (whose approval
shall not be unreasonably withheld, conditioned or delayed); and, provided,
further, that the failure of any Indemnified Party to give notice as provided
herein shall not relieve the Indemnifying Party of its obligations under this
Section 7 except to the extent that the Indemnifying Party is adversely affected
by such failure. The Indemnified Party may participate in such defense at such
party's expense; provided, however, that the Indemnifying Party shall pay such
expense if the Indemnified Party reasonably concludes based upon written advice
of its counsel that representation of such Indemnified Party by the counsel
retained by the Indemnifying Party would be inappropriate due to actual or
potential differing interests between the Indemnified Party and any other party
represented by such counsel in such proceeding; provided further that in no
event shall the Indemnifying Party be required to pay the expenses of more than
one law firm per jurisdiction as counsel for the Indemnified Party. The
Indemnifying Party also shall be responsible for the expenses of such defense if
the Indemnifying Party does not elect to assume such defense. No Indemnifying
Party, in the defense of any such claim or litigation shall, except with the
consent of each Indemnified Party, consent to entry of any judgment or enter
into any settlement which does not include as an unconditional term thereof the
giving by the claimant or plaintiff to such Indemnified Party of a release from
all liability in respect of such claim or litigation, and no Indemnified Party
shall consent to entry of any judgment or settle such claim or litigation
without the prior written consent of the Indemnifying Party, which consent shall
not be unreasonably withheld, conditioned or delayed.

          (d) In order to provide for just and equitable contribution in
circumstances in which the indemnification provided for in this Section 7 is due
in accordance with its terms but for any reason is held to be unavailable to an
Indemnified Party in respect to any losses, claims, damages and liabilities
referred to herein, then the Indemnifying Party shall, in lieu of indemnifying
such Indemnified Party, contribute to the amount paid or payable by such


                                       6
<PAGE>
Indemnified Party as a result of such losses, claims, damages or liabilities to
which such party may be subject in such proportion as is appropriate to reflect
the relative fault of the Company on the one hand and the Rightsholders on the
other in connection with the statements or omissions which resulted in such
losses, claims, damages or liabilities, as well as any other relevant equitable
considerations. The relative fault of the Company and the Rightsholders shall be
determined by reference to, among other things, whether the untrue or alleged
untrue statement of material fact related to information supplied by the Company
or the Rightsholders and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such statement or omission.
The Company and the Rightsholders agree that it would not be just and equitable
if contribution pursuant to this Section 7(d) were determined by pro rata
allocation or by any other method of allocation which does not take account of
the equitable considerations referred to above. Notwithstanding the provisions
of this Section 7(d), (i) in no case shall any one Rightsholder be liable or
responsible for any amount in excess of the gross proceeds received by such
Rightsholder from the offering of Registrable Securities and (ii) the Company
shall be liable and responsible for any amount in excess of such proceeds;
provided, however, that no person guilty of fraudulent misrepresentation (within
the meaning of Section 11(f) of the Securities Act) shall be entitled to
contribution from any person who was not guilty of such fraudulent
misrepresentation. Any party entitled to contribution will, promptly after
receipt of notice of commencement of any action, suit or proceeding against such
party in respect of which a claim for contribution may be made against another
party or parties under this Section 7(d), notify such party or parties from whom
contribution may be sought, but the omission so to notify such party or parties
from whom contribution may be sought shall not relieve such party from any other
obligation it or they may have thereunder or otherwise under this Section 7(d)
except to the extent that the party or parties from whom contribution may be
sought are adversely affected. No party shall be liable for contribution with
respect to any action, suit, proceeding or claim settled without its prior
written consent, which consent shall not be unreasonably withheld, conditioned
or delayed.

     8. Reporting. With a view to making available to the Rightsholders the
benefits of Rule 144 promulgated under the Securities Act or any other similar
rule or regulation of the Commission that may at any time permit the
Rightsholders to sell securities of the Company to the public without
registration ("Rule 144"), for so long as Rightsholders continue to own
Registrable Securities, the Company shall use its reasonable efforts to:

          (a) make and keep public information available, as those terms are
understood and defined in Rule 144, and file with the Commission in a timely
manner all reports and other documents required of the Company under the
Securities Act and the Exchange Act; and

          (b) furnish to each Rightsholder, for so long as such Rightsholder
owns Registrable Securities, promptly upon request, (i) a written statement by
the Company, if true, that it has complied with the applicable reporting
requirements of Rule 144, the Securities Act and the Exchange Act, (ii) a copy
of the most recent annual or quarterly report of the Company and (iii) such
other information as may be reasonably requested to permit the Rightsholders to
sell such securities pursuant to Rule 144 without registration.

     9. Assignment of Registration Rights. The rights under this Agreement shall
not be assigned by any Rightsholder except in connection with the transfer of
Registrable Securities by


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<PAGE>
such Rightsholder to an affiliate of such Rightsholder, provided that (i) the
Rightsholder agrees in writing with the transferee or assignee to assign such
rights, and a copy of such agreement is furnished to the Company; (ii) the
Company is furnished with written notice of (a) the name and address of such
transferee or assignee, and (b) the securities with respect to which such rights
are being transferred or assigned; (iii) at or before the time the Company
receives the written notice contemplated by clause (ii) of this sentence, the
transferee or assignee agrees in writing with the Company to be bound by all of
the obligations of an Rightsholder under this Agreement; and (iv) such transfer
shall have been conducted in accordance with all applicable federal and state
securities laws.

     10. Amendment of Registration Rights.

          (a) Any provision of this Agreement may be amended and the observance
of any provision of this Agreement may be waived (either generally or in a
particular instance and either retroactively or prospectively), only with the
written consent of the Company and Rightsholders who then hold at least a
majority of the Registrable Securities. Any amendment or waiver effected in
accordance with this Section 10 shall be binding upon each Rightsholder and the
Company. No such amendment shall be effective to the extent that it applies to
less than all of the holders of the Registrable Securities. No consideration
shall be offered or paid to any Person to amend or consent to a waiver or
modification of any provision of any of this Agreement unless the same
consideration also is offered to all of the parties to this Agreement.

          (b) In the event that the Company issues and sells Notes as part of
the Offering after the date hereof, the Company shall have the right to amend
this Agreement without the consent of the Rightsholders to include the
purchasers of such Notes in this Agreement as Purchasers and Rightsholders and
any placement agent or selected dealer that receives warrants in connection with
the sale of such Notes as an Agent and Rightsholder and in connection therewith
to modify the Schedule of Purchasers to include such Purchaser and the Agent
Schedule to include such Agent.

     11. Miscellaneous.

          (a) A Person is deemed to be a holder of Registrable Securities
whenever such Person owns or is deemed to own of record such Registrable
Securities. If the Company receives conflicting instructions, notices or
elections from two or more Persons with respect to the same Registrable
Securities, the Company shall act upon the basis of instructions, notice or
election received from the record owner of such Registrable Securities.

          (b) Any notices, consents, waivers or other communications required or
permitted to be given under the terms of this Agreement must be in writing and
will be deemed to have been delivered: (i) upon receipt, when delivered
personally; (ii) upon receipt, when sent by facsimile; or (iii) two (2) Business
Days after deposit with a reputable overnight delivery service, in each case
properly addressed to the party to receive the same. The addresses and facsimile
numbers for such communications shall be:

     If to the Company:


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<PAGE>
                       Hybridon, Inc.
                       345 Vassar Street
                       Cambridge, MA 02139-4818
                       Telephone: 617-679-5500
                       Facsimile: 617-679-5592
                       Attention: Chief Financial Officer

     with a copy to:

                       Wilmer Cutler Pickering Hale and Dorr LLP
                       60 State Street
                       Boston, MA 02109
                       Telephone: 617-526-6000
                       Facsimile: 617-526-5000
                       Attention: David E. Redlick, Esq.

     If to a Rightsholder, to its address and facsimile number set forth on the
Schedule of Purchasers or on the Agent Schedule, as the case may be, or to such
other address and/or facsimile number and/or to the attention of such other
Person as the recipient party has specified by written notice given to each
other party five (5) days prior to the effectiveness of such change.

     Written confirmation of receipt (A) given by the recipient of such notice,
consent, waiver or other communication, (B) mechanically or electronically
generated by the sender's facsimile machine containing the time, date, recipient
facsimile number and an image of the first page of such transmission, or (C)
provided by a courier or overnight courier service shall be rebuttable evidence
of personal service, receipt by facsimile or receipt from a reputable overnight
delivery service in accordance with clause (i), (ii) or (iii) above,
respectively.

          (c) Failure of any party to exercise any right or remedy under this
Agreement or otherwise, or delay by a party in exercising such right or remedy,
shall not operate as a waiver thereof.

          (d) All questions concerning the construction, validity, enforcement
and interpretation of this Agreement shall be governed by the internal laws of
the State of Delaware, without giving effect to any choice of law or conflict of
law provision or rule (whether of the State of Delaware or any other
jurisdictions) that would cause the application of the laws of any jurisdictions
other than the State of Delaware.

          (e) This Agreement and the documents referenced herein constitute the
entire agreement among the parties hereto with respect to the subject matter
hereof and thereof. There are no restrictions, promises, warranties or
undertakings, other than those set forth or referred to herein and therein. This
Agreement and the documents referenced herein supersede all prior agreements and
understandings among the parties hereto with respect to the subject matter
hereof.


                                       9
<PAGE>
          (f) Subject to the requirements of Section 9 of this Agreement, this
Agreement shall inure to the benefit of and be binding upon the permitted
successors and assigns of each of the parties hereto.

          (g) The headings in this Agreement are for convenience of reference
only and shall not limit or otherwise affect the meaning hereof.

          (h) This Agreement may be executed in identical counterparts, each of
which shall be deemed an original but all of which shall constitute one and the
same agreement. This Agreement, once executed by a party, may be delivered to
the other parties hereto by facsimile transmission of a copy of this Agreement
bearing the signature of the party so delivering this Agreement.

          (i) Each party shall do and perform, or cause to be done and
performed, all such further acts and things, and shall execute and deliver all
such other agreements, certificates, instruments and documents, as the other
party may reasonably request in order to carry out the intent and accomplish the
purposes of this Agreement and the consummation of the transactions contemplated
hereby.

          (j) All consents and other determinations required to be made by the
Rightsholders pursuant to this Agreement shall be made, unless otherwise
specified in this Agreement, by Rightsholders holding at least a majority of the
Registrable Securities.

          (k) The language used in this Agreement will be deemed to be the
language chosen by the parties to express their mutual intent and no rules of
strict construction will be applied against any party.

          (l) This Agreement is intended for the benefit of the parties hereto
and their respective permitted successors and assigns, and is not for the
benefit of, nor may any provision hereof be enforced by, any other Person.

                  [REMAINDER OF PAGE LEFT BLANK INTENTIONALLY]


                                       10
<PAGE>
     IN WITNESS WHEREOF, the parties have caused this Registration Rights
Agreement to be duly executed as of the day and year first above written.

                                        COMPANY:

                                        HYBRIDON, INC.


                                        By: /s/ R. Andersen
                                            ------------------------------------
                                        Name: R.G. Andersen
                                        Title: CFO


                                        PURCHASERS:

                                        Counterpart signature pages attached.


                                        AGENT:

                                        Counterpart signature pages attached.


                                       11
<PAGE>
                                    Exhibit A

                             Schedule of Purchasers

<TABLE>
<CAPTION>
Name and Address               Registrable Securities
----------------               ----------------------
<S>                            <C>
Abdel Raouf M. Abou Anza                 89,888
Hyb Invest I ApS                        898,876
Optima Life Sciences Limited          3,486,236
Saleh Abdullah Alattas                1,179,775
</TABLE>
<PAGE>
Exhibit B

                                 Agent Schedule

<TABLE>
<CAPTION>
Name and Address             Registrable Securities
----------------             ----------------------
<S>                          <C>
Pillar Investments Limited           565,478
</TABLE>