Exhibit 5
                                                                       ---------

                  [Testa, Hurwitz & Thibeault, LLP Letterhead]


                                                            April 9, 1999


Repligen Corporation
117 Fourth Avenue
Needham, Massachusetts 02194

        Re:    S-3 Registration Statement
               --------------------------

Ladies and Gentlemen:

     We are counsel to Repligen Corporaton, a Delaware corporation (the
"Company"), and have represented the Company in connection with the preparation
and filing of the Company's Form S-3 Registration Statement (the "Registration
Statement"), covering the sale to the public of up to 262,500 shares of the
Company's Common Stock, $.01 par value per share (the "Shares"), being sold by a
stockholder of the Company.

     We have reviewed the corporate proceedings taken by the Board of Directors
of the Company with respect to the authorization and issuance of the Shares. We
have also examined and relied upon originals or copies, certified or otherwise
authenticated to our satisfaction, of all corporate records, documents,
agreements or other instruments of the Company and have made all investigations
of law and have discussed with the Company's officers all questions of fact that
we have deemed necessary or appropriate.

     Based upon and subject to the foregoing, we are of the opinion that the
Shares are legally issued, fully paid and non-assessable.

     We hereby consent to the filing of this opinion as Exhibit 5 to the
Registration Statement and to the reference to our firm in the Prospectus
contained in the Registration Statement under the caption "Legal Matters."



                                             Very truly yours,

                                             /s/ TESTA, HURWITZ & THIBEAULT, LLP
                                             TESTA, HURWITZ & THIBEAULT, LLP