EXHIBIT 5

                       KRAMER LEVIN NAFTALIS & FRANKEL LLP

                                919 THIRD AVENUE

                           NEW YORK, N.Y. 10022 - 3852


   TEL (212) 715-9100                                      47, Avenue Hoche
   FAX (212) 715-8000                                         75008 Paris
                                                                France

                                 March 31, 2000


Hybridon, Inc.
155 Fortune Boulevard
Milford, Massachusetts  01757

                  Re:   Registration Statement on Form S-8

Ladies and Gentlemen:

            We have acted as counsel to Hybridon, Inc., a Delaware corporation
(the "Registrant"), in connection with the preparation and filing of a
Registration Statement on Form S-8 (the "Registration Statement") with the
Securities and Exchange Commission (the "Commission"), with respect to the
registration under the Securities Act of 1933, as amended (the "Act"), of an
aggregate of 6,500,000 shares (the "Shares") of common stock, par value $0.001
per share ("Common Stock"), to be issued pursuant to the Registrant's Hybridon,
Inc. 1997 Stock Incentive Plan, as amended (the "Plan").

            In connection with the registration of the Shares, we have reviewed
copies of the Registration Statement, the Plan, the Restated Certificate of
Incorporation, as amended, of the Registrant and the Amended and Restated
By-laws of the Registrant, and such documents and records as we have deemed
necessary to enable us to express an opinion on the matters covered hereby.

            We have also examined and relied upon representations, statements,
or certificates of public officials and officers and representatives of the
Registrant.

            Based upon the foregoing, we are of the opinion that the Shares
covered by the Registration Statement, following the granting of the options and
other stock-based awards described in the Plan and upon delivery of such Shares
and payment therefor at the prices and in accordance with the terms stated in
the Plan will be validly issued, fully paid and non-assessable.

            We hereby consent to the use of this opinion as an exhibit to the
Registration Statement. In giving the foregoing consent, we do not thereby admit
that we are in the category of persons whose consent is required under Section 7
of the Act or the rules and regulations of the Commission thereunder.

            We are delivering this opinion to the Registrant, and no person
other than the Registrant may rely upon it.

                                    Very truly yours,



                                    /s/ Kramer Levin Naftalis & Frankel LLP