[letterhead of Testa, Hurwitz & Thibeault, LLP]

                                                                January 28, 2000

Repligen Corporation
117 Fourth Avenue
Needham, Massachusetts 02194

     Re:   S-3 REGISTRATION STATEMENT

Ladies and Gentlemen:

     We are counsel to Repligen Corporation, 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 2,007,050 shares of the
Company's Common Stock, $.01 par value per share (the "Shares"), upon the
exercise of the warrants described in the Registration Statement (the
"Warrants").

     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 upon
exercise of the Warrants. 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, when issued upon the exercise of the Warrants in accordance with the
terms of the Registration Statement, will be 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