Exhibit 99.1

                                                    Repligen Corporation
                                                    41 Seyon Street
RepliGen                                            Building #1, Suite 100
                                                    Waltham, Massachusetts 02453
                                                    Telephone: 781-250-0111
FOR IMMEDIATE RELEASE                               Telefax:   781-250-0115
- --------------------------------------------------------------------------------

CONTACT:
Walter C. Herlihy, Ph.D.                                 David A. Walsey
President and Chief Executive Officer                    The Ruth Group
(781) 250-0111, ext. 2000                                (646) 536-7029

              Repligen Reports First Quarter 2004 Financial Results

WALTHAM, MA - July 31, 2003 - Repligen Corporation (Nasdaq: RGEN) today reported
results for the first quarter of fiscal year 2004 ended June 30, 2003. Total
revenue for the quarter was $2,061,000 compared to total revenue of $1,619,000
for the first quarter of fiscal year 2003, an increase of 27%. Total revenue for
the first quarter consisted of rProtein A(TM) sales, SecreFlo(TM) sales, a
product for pancreatic assessment that was launched in October 2002, and
research revenue. Gross profit for the first quarter was $1,205,000, or 59% of
revenue compared to $949,000, or 59% of revenue for the first quarter of fiscal
year 2003.

Operating expenses for the first quarter of fiscal year 2004 ended June 30, 2003
were $3,331,000 compared to $2,109,000 for the first quarter of fiscal year
2003. The net loss for the first quarter of fiscal year 2004 ended June 30, 2003
was $2,028,000 or $.07 per share, compared to $991,000 or $.04 per share for the
first quarter of fiscal year 2003. Cash and investments as of June 30, 2003 were
$29,395,000.

"We made significant progress this quarter in advancing our clinical candidates
and improving our financial strength," stated Walter C. Herlihy, President and
Chief Executive Officer of Repligen. "As we look forward to the next six months,
we expect results from our lead Phase 3 clinical trial of secretin in autism,
initiation of our Phase 2 clinical trial of secretin in schizophrenia and
potential progress in developing our patent position for CTLA4-Ig."

Quarterly Highlights

      o     We completed enrollment in our lead Phase 3 clinical trial of
            secretin for autism in May. The last patient will have their last
            visit in November and the results are anticipated by the end of the
            year. All 17 of the Phase 3 sites have expressed interest in
            participating in the Phase 3 extension study which involves dosing
            with secretin once every three weeks for a period of a year and will
            enable us to collect valuable long term safety data. The extension
            study is being conducted at our election and will not impact the
            timeline of the Phase 3 study.

      o     We plan to submit a Phase 2 clinical trial protocol to the FDA in
            September for schizophrenia targeting the social deficits ("negative
            symptoms") characteristic of many patients with schizophrenia. This
            double blind, placebo-controlled study will target patients who are
            refractory to the current standard of care and is expected to
            initiate later this year at multiple centers in the United States.
            There are approximately 2 million people in the United States with
            schizophrenia.

                                     -more-



Repligen Reports First Quarter 2004 Financial Results, July 31, 2003
Page 2 of 3

      o     The CTLA4-Ig trial against Bristol-Myers Squibb for correction of
            inventorship of certain patents relating to CTLA4-Ig is complete and
            we are currently awaiting a ruling from the Court. Repligen and the
            University of Michigan have accused Bristol-Myers Squibb of failing
            to name a University scientist as an inventor on the CTLA4 patents
            issued to Bristol and of engaging in fraudulent conduct to obtain
            certain of the patents.

      o     Repligen was added to the Russell 2000(R) Index, which is determined
            by objective rules, such as market capitalization rankings which
            will remain in place for a year. Russell indexes are used by
            investment managers for index funds and as benchmarks for both
            passive and active strategies. About $220 billion is invested in
            index funds based on Russell's indexes and an additional $850
            billion is benchmarked to them. Investment managers who oversee
            these funds purchase shares of member stocks according to that
            company's weighting in the particular index.

      o     We completed a private placement for gross proceeds of $12.5
            million, which will enable us to expand our efforts to develop
            secretin for autism and schizophrenia, to increase our investment in
            the process development and manufacturing of CTLA4-Ig and for
            working capital, capital expenditures and general corporate
            purposes.

      o     We entered into a binding term sheet for settlement of the
            Pro-Neuron complaint related to triacetyl uridine, which will be
            dismissed upon execution of definitive agreements. Under the terms
            of the settlement, Repligen will receive $750,000 and the UCSD
            License Agreements will be revised to exclude the field of acylated
            pyrimidines. We will continue our clinical trials of TAU in bipolar
            disorder/major depression and purine autism for up to two years and
            will discontinue our clinical trial of TAU in mitochondrial disease.

      o     Researchers from Duke University Medical Center reported that
            SecreFlo(TM) reduced the incidence of pancreatitis in patients
            undergoing an invasive diagnostic procedure called ERCP. Analysis of
            patient subgroups indicated that the prevention of pancreatitis
            after ERCP was particularly significant in those patients undergoing
            a surgical procedure involving the bile duct.

Quarterly Conference Call

Repligen's President and Chief Executive Officer, Walter C. Herlihy, Ph.D., will
host a conference call and webcast on Friday, August 1st at 11 a.m. EDT, to
provide a quarterly update of the Company. This call can be accessed at
www.repligen.com or you may listen to the live broadcast by calling (800)
915-4836 for domestic calls and (973) 317-5319 for international calls.

About Repligen Corporation

Repligen Corporation is a biopharmaceutical company committed to being the
leader in the development of new drugs for pediatric developmental disorders
including autism, immune and metabolic disorders. Repligen has a Specialty
Pharmaceuticals business comprised of rProtein A(TM) and SecreFlo(TM), the
profits from which will be used to support the development of our proprietary
products. rProtein A(TM) is a consumable reagent used by the pharmaceutical
industry to produce a class of drugs called monoclonal antibodies and
SecreFlo(TM), secretin for injection, is marketed to gastroenterologists for
pancreatic assessment and for use during a gastrointestinal procedure called
ERCP. Repligen's corporate headquarters are located at 41 Seyon Street, Building
#1, Suite 100, Waltham, MA 02453. Additional information may be requested from
www.repligen.com.

                                     -more-



Repligen Reports First Quarter 2004 Financial Results, July 31, 2003
Page 3 of 3

                             SELECTED FINANCIAL DATA

Operating Statement Data:

                                                       Three-months ended
                                                            June 30,

                                                     2003              2002
                                                 ------------------------------
Revenue:
     Product revenue                             $  2,043,000      $  1,619,000
     Research revenue                                  18,000                --
                                                 ------------      ------------
Total revenue                                       2,061,000         1,619,000
Cost of revenue                                       856,000           670,000
                                                 ------------      ------------
     Gross profit                                   1,205,000           949,000
Operating expenses:
     Research and development                       1,428,000         1,227,000
     Selling, general and administrative            1,903,000           882,000
                                                 ------------      ------------
Total operating expenses                            3,331,000         2,109,000

Loss from operations                               (2,126,000)       (1,160,000)
Investment income                                      98,000           169,000
Net loss                                         $ (2,028,000)     $   (991,000)
                                                 ============      ============

Basic and diluted net loss per share             $       (.07)     $       (.04)
                                                 ============      ============

Basic and diluted weighted average
shares outstanding                                 28,987,000        26,643,000
                                                 ============      ============

Balance Sheet Data:

                                                 June 30, 2003    March 31, 2003
                                                 -------------    --------------
Cash and investments                             $ 29,395,000      $ 18,909,000

Total assets                                       36,806,000        26,793,000

Stockholders' equity                               34,433,000        24,550,000

This release contains forward-looking statements which are made pursuant to the
safe harbor provisions of Section 27A of the Securities Act of 1933 and Section
21E of the Securities Exchange Act of 1934. The forward-looking statements in
this release do not constitute guarantees of future performance. Investors are
cautioned that statements in this press release which are not strictly
historical statements, including, without limitation, statements regarding
current or future financial performance, management's strategy, litigation,
plans and objectives for future operations, clinical trials and results and
product development and manufacturing plans and performance such as the
anticipated growth in the monoclonal antibody market and projected growth in
product sales, constitute forward-looking statements. Such forward-looking
statements are subject to a number of risks and uncertainties that could cause
actual results to differ materially from those anticipated, including, without
limitation, risks associated with: the success of current and future
collaborative relationships, the success of our clinical trials and our ability
to develop and commercialize products, our ability to obtain required regulatory
approvals, our compliance with all Food and Drug Administration regulations, our
ability to obtain, maintain and protect intellectual property rights for our
products, the risk of current and future litigation regarding our patent and
other intellectual property rights, the risk of litigation with collaborative
partners, our limited sales and marketing experience and capabilities, our
limited manufacturing capabilities and our dependence on third-party
manufacturers and value-added resellers, our ability to hire and retain skilled
personnel, the market acceptance of our products, our ability to compete with
larger, better financed pharmaceutical and biotechnology companies that may
develop new approaches to the treatment of our targeted diseases, our history of
losses and expectation of incurring continued losses, our ability to generate
future revenues, our ability to raise additional capital to continue our drug
development programs, our volatile stock price, the effects of our anti-takeover
provisions, and other risks detailed in Repligen's filings with the Securities
and Exchange Commission. Repligen assumes no obligation to update any
forward-looking information contained in this press release or with respect to
the announcements described herein.

                                       ###