SecProbe.io

Mail Stop 4720                                                                                                   July 14, 2009   Walter C. Herlihy, Ph.D. President, Chief Executive Officer and Director 41 Seyon Street, Building #1, Suite 100 Waltham, Massachusetts 02453
Re: Repligen Corporation
  Supplemental Response to Form 10-K for the Year
Ended March 31, 2008
  Filed May 29, 2009   File No. 000-14656

Dear Dr. Herlihy:

We have reviewed your filing and have th e following comments.  In our comments,
we ask you to provide us with information to  better understand your disclosure.  Where it
requests you to revise disclosure, the information you provide should show us what the revised disclosure will look like and identify the annual or quarterly filing, as applicable, in
which you intend to first include  it.  If you do not believe th at revised disclosure is
necessary, explain the reason in your response.  After reviewing the information provided,
we may raise additional comments and/ or request that you amend your filing.
  Please understand that the purpose of our re view process is to assist you in your
compliance with the applicable disclosure requirements and to enhance the overall disclosure in your filing.  We look forward to  working with you in these respects.  We
welcome any questions you may have about our comments or on any other aspect of our review.  Feel free to call us at the telephone numbers listed at the end of this letter.

Form 10-K for the year ended March 31, 2008

Item 1. Business, page 3
1. Please provide us with an analysis supporting your determination that you were not substantially dependent on your agreemen t with Applied as of March 31, 2008.
Your analysis should focus on the facts and circumstances as of March 31, 2008.  If you had reason to believe that your depende nce of this agreement would decline
after March 31, 2008, this information ma y be relevant but would not be
determinative.
2. We note your response to our prior comment 2.  Please provide proposed disclosure

Walter C. Herlihy, Ph.D.
Repligen Corporation July 14, 2009 Page 2
 to include information regarding the expiration of the agreement and each party’s termination rights thereunder.  We note that your Form 10-K for the fiscal year
ended March 31, 2009 filed June 11, 2009 doe s not contain this information.

*    *    *    *

Please provide us the information requested within 10 business days or tell us when
you will provide us with a response.  Please furnish a cover letter with your response that keys your responses to our comments and provide any requested information.  Detailed cover letters greatly facilita te our review.  Please furnis h your letter on EDGAR under the
form type label CORRESP.

 We urge all persons who are responsible for the accuracy and adequacy of the
disclosure in the filing to be certain that the filing includes all in formation required under
the Securities Exchange Act of 1934 and that they have provided all information investors
require for an informed investment decision.  Since the company and its management are
in possession of all facts relating to a company’s disclosure, they are responsible for the accuracy and adequacy of the disclosures they have made.     In connection with responding to our co mments, please provide, in your letter, a
statement from the company acknowledging that:

• the company is responsible for the adequacy  and accuracy of the disclosure in the
filing;
• staff comments or changes to disclosure  in response to staff comments do not
foreclose the Commission from taking any action with respect to the filing; and
• the company may not assert staff comments as a defense in any proceeding initiated
by the Commission or any person under the federal securities laws of the United
States.

In addition, please be advise d that the Division of Enfo rcement has access to all
information you provide to the staff of the Divi sion of Corporation Fi nance in our review
of your filing or in response to our comment on your filing.
Please contact Laura Crotty, Staff Attorn ey, at (202) 551-3563 or Suzanne Hayes,
Legal Branch Chief, at (202) 551-3675 if you ha ve questions regardi ng the processing of
your response as well as any questions regarding co mments and related matters.
 Sincerely,

Jeffrey P. Riedler Assistant Director