Agreement

Exhibit 10.1

Agreement

THIS AGREEMENT (the “Agreement”) is made and entered into as of May 4, 2012, by and between Ronald L. Chez (together with any affiliate thereof, the “Stockholder”) and Repligen Corporation, a Delaware corporation (the “Company”).

Recitals

WHEREAS, the Stockholder submitted a proposal to the Company under Rule 14a-8 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), regarding the adoption of an amendment to the Amended and Restated By-Laws of the Company which would lower the threshold necessary for calling a special meeting of stockholders from a majority to the holders of 20% of the outstanding voting shares of the Company (the “Proposal”);

WHEREAS, pursuant to Rule 14a-8 of the Exchange Act, the Company included the Proposal in the Company’s definitive 2012 proxy statement and form of proxy (together, the “2012 Proxy Materials”) filed with the Securities and Exchange Commission (the “SEC”) on April 20, 2012 and first sent or given to the Company’s stockholders on or about April 23, 2012; and

WHEREAS, the Stockholder and the Company desire to enter into this Agreement pursuant to which, among other things, (i) the Stockholder agrees to amend and restate the Proposal in the form attached hereto as Exhibit A (the “Amended and Restated Proposal”), (ii) the Company’s Board of Directors (the “Board”) agrees to recommend that the Company’s stockholders vote “FOR” the Amended and Restated Proposal and (iii) the Stockholder agrees to vote all of the voting shares of the Company held or controlled by the Stockholder (collectively, the “Shares”) in accordance with the recommendation of the Board with respect to each of the proposals to be acted upon at the Company’s 2012 Annual Meeting of Stockholders (the “2012 Annual Meeting”).

Agreement

NOW, THEREFORE, in consideration of these premises and the covenants contained herein, the parties hereto, each intending to be legally bound, hereby agree as follows:

If to the Company:

    Repligen Corporation

    41 Seyon Street, Building #1, Suite 100

    Waltham, Massachusetts 02453

    Attention: Walter C. Herlihy, President and Chief Executive Officer

    Fax No. (781)  ###-###-####

    Email: ***@***

With a copy (which shall not constitute notice) to:

    Goodwin Procter LLP

    53 State Street

    Boston, Massachusetts 02109

    Attention: Joseph L. Johnson III and Arthur R. McGivern

    Fax No. (617)  ###-###-####

    Email: ***@***

                 ***@***

If to the Stockholder:

    Ronald L. Chez

    1524 N. Astor Street

    Chicago, Illinois 60610

    Email: ***@***

With a copy (which shall not constitute notice) to:

    Thompson Coburn LLP

    55 East Monroe Street, 37^th Floor

    Chicago, Illinois 60603

    Attention: Barry Fischer

    Fax No. (312)  ###-###-####

    Email: ***@***

[Remainder of page intentionally left blank]

IN WITNESS WHEREOF, each of the parties hereto has executed this Agreement, or caused the same to be executed by its duly authorized representative as of the date first above written.

[Signature Page to Agreement with Ronald Chez]

EXHIBIT A

Amended and Restated Proposal

Lowered Voting Threshold to Call Special Meetings of Shareholders

RESOLVED, that the first sentence of Article I, Section 3 of the Amended and Restated By-Laws of Repligen Corporation be amended and restated to read as follows:

“Special meetings of the stockholders may be called at any time by the President, the Chairman or the Board of Directors; and shall be called by the Secretary or any officer upon the written request of one or more stockholders holding, in the aggregate, at least 30% of the outstanding shares of stock of the corporation entitled to vote at such meeting.”

The purpose of this proposal is to lower the threshold necessary for calling a special meeting of shareholders to the holders of 30% of voting shares. Currently, no single shareholder holds more than 13.9% of the company’s voting stock.

At present, calling a special stockholder meeting requires the consent of the holders of over 50% of Repligen’s voting stock. Meanwhile, the Board of Directors, whose nonexecutive directors (according to Repligen’s 2011 Proxy Statement) hold less than 3.5% of the company’s outstanding stock (excluding options), can call a special stockholder meeting at any time.

Special meetings allow for increased shareholder involvement in important matters, including electing new directors. Shareholder participation in Repligen’s affairs is also important as the company transitions from a drug development company to an operating company, including potential issues such as executive compensation criteria, stockholder ownership of Board members and officers, assuring that the composition of the Board is consistent with an operating company and other issues.

Without the ability for shareholders to call special meetings, directors and management can become insulated. Repligen’s shareholders want to enhance the alignment of the Board’s and management’s interests with those of its shareholders, all in the interest of Repligen’s performance and shareholder value. Approving this proposal will send a clear message to Repligen’s directors that they must be accountable and responsive to Repligen’s shareholders.

Many public companies have reduced their special meetings requirement from a majority requirement. Pfizer, AT&T Inc., PepsiCo, Inc., Caterpillar Inc., Honeywell International and other companies require only the holders of 20% or less of its stock to call a special meeting. This proposal topic won more than 60% support at CVS Caremark, Sprint Nextel, Safeway, Motorola and R.R. Donnelley.

Please vote yes on this proposal to help improve Repligen’s corporate governance, director and officer accountability and financial performance that Repligen’s shareholders deserve.

EXHIBIT B

Additional Soliciting Materials

EXHIBIT C

Schedule 13D/A

EXHIBIT D

Press Release