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                                                                                                 April 16, 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 March 31, 2009   File No. 000-14656

Dear Dr. Herlihy:

We have reviewed your filing and have the following comments.  In our
comments, we ask you to provide us with  information to better understand your
disclosure.  Where it requests you to revi se 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 that 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 year ended March 31, 2008

Item 1. Business, page 3
1. a. In your response to our prior comment 1 you state that Applied’s purchases of
Protein A from the Company account for “less 10% of the Company’s product
revenues and less than 4% of total revenues.”  However, we note that your
disclosure on page 5 of the 10-K under “Significant Customers” states that
Applied was one of two customers whose purchases of Protein A products

Walter C. Herlihy, Ph.D.
Repligen Corporation
April 16, 2009 Page 2
accounted for more than 10% of the Company’s total revenues in 2008, 2007
and 2006.  Please explain this apparent discrepancy.
b. In addition, you state that the supply ag reement does not obligate the parties to
sell or purchase specific qua ntities of Protein A.  Yet, you also state that
Repligen and Applied are in fact obligat ed with respect to the quantities of
Protein A reflected in Applied’s short-term forecasts of its Protein A needs and these amounts are not fixed.  So, while the amounts of Protein A under
the supply agreement may have been imma terial so far, it seems possible that
they could become much larger in the future.  If true, while the agreement might qualify as immaterial right no w, depending on the revenues involved
thus far, it could become material in th e future.  Please inform us whether this
is the case and, if so, that you will file the agreement as a material contract
should revenues attributable to the contract become material.
2. We note your response to our prior comment  3.  Please also provide the following
material information regarding the ex clusive license agreement between the
company and the University of Michigan:
a. Total amounts received to date under the agreement;
b. Any annual fees, if applicable;
c. The number of additional products you anticipate you will develop under
this agreement; and
d. The term and termination provisions, including any payments the company
would be required to make in  the event of termination.

Definitive Proxy Statement

Annual Cash Incentive Compensation, page 19
3. We note your response to our prior comment  8.  Please supplementally provide us
with a detailed analysis of why you believe public disc losure of the requested
information, based upon your specific facts and circumstances, would cause the
company competitive harm.  Your analysis  in your response letter should clearly
describe each target, describe  how it is used in the cal culation of performance,
and explain how such information is likel y to cause competitive harm, as opposed
to conclusory statements that the in formation would represent a competitive
disadvantage.  Unless you disclose this in formation to the staff it is not possible
for us to determine whether omission of the information from your 10-K disclosure is appropriate.  Please note that you may re quest Rule 83 confidential

Walter C. Herlihy, Ph.D.
Repligen Corporation
April 16, 2009 Page 3
treatment for portions of your response le tter that contain co mmercially sensitive
information.
In the alternative, please provide complete disclosure  of the specific targets
against which company and individual  performance was measured and the
calculations used to determine and s ubsequently measure performance. The
disclosure should allow the reader to understand how the Compensation
Committee determined the percentage of achievement of individual and corporate
goals.

*    *    *    *

Please provide us the information request ed within 10 busine ss 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 facilitate our  review.  Please furnish 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 th at they have provided all information
investors require for an informed invest ment decision.  Since the company and its
management are in possession of all facts re lating 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 comme nts as a defense in any proceeding
initiated by the Commission or any person under the federal secu rities 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.

Walter C. Herlihy, Ph.D.
Repligen Corporation April 16, 2009 Page 4
 Please contact Laura Crotty, Staff Atto rney, at (202) 551-3563, Dan Greenspan,
Special Counsel, at (202) 551-36231 or myself at (202) 551-3715 if you have questions regarding the processing of your response as well as any questions regarding comments
and related matters.

Sincerely,

Jeff Riedler Assistant Director