Exhibit No. 99.1

                      [Letterhead of RepliGen Corporation]

FOR IMMEDIATE RELEASE
- --------------------------------------------------------------------------------
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 Second Quarter 2004 Financial Results

WALTHAM, MA - November 12, 2003 - Repligen Corporation (NASDAQ: RGEN) today
reported results for the second quarter of fiscal year 2004 ended September 30,
2003. Total revenue for the quarter was $1,419,000 compared to total revenue of
$1,687,000 for the second quarter of fiscal year 2003. Gross profit for the
quarter was $678,000, or 48% of revenue compared to $1,025,000, or 61% of
revenue for the second quarter of fiscal year 2003.

Operating expenses for the second quarter of fiscal year 2004 ended September
30, 2003 were $2,912,000 compared to $2,264,000, for the second quarter in
fiscal year 2003. The net loss for the second quarter of fiscal year 2004 was
$2,140,000 or $.07 per share, compared to $1,083,000 or $.04 per share for the
second quarter of fiscal year 2003. Cash and investments as of September 30,
2003 were $27,754,000.

"We have made significant progress this quarter in advancing our secretin
development programs and obtaining a better understanding of secretin's activity
in the brain," stated Walter C. Herlihy, President and Chief Executive Officer
of Repligen Corporation. "We look forward to presenting the forthcoming results
of our first Phase 3 trial in autism."

For the six-month period ended September 30, 2003, total revenues were
$3,480,000 compared to $3,307,000 for the same period in fiscal year 2003. For
the six-month period ended September 30, 2003, gross profit was $1,884,000, or
54% of revenue compared to $1,976,000, or 60% of revenue for the same period in
fiscal year 2003.

Year-to-date operating expenses for the six-month period ended September 30,
2003 were $6,244,000 compared to $4,374,000 for the same period in fiscal year
2003. For the six-month period ended September 30, 2003, the net loss was
$4,168,000 or $.14 per share compared to $2,074,000 or $.08 per share for the
same period in fiscal year 2003.

Recent Highlights


      |X|   The last patient in our first Phase 3 clinical trial of secretin in
            autism will have their last visit this month and the top line
            results are anticipated at the end of the year. The primary


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Repligen Reports Second Quarter 2004 Financial Results, November 12, 2003
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            endpoints of the study are improvements in social interaction as
            measured by the Autism Diagnostic Observation Schedule (ADOS) and
            Clinical Global Impression of Change (CGI).

      |X|   All 15 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.

      |X|   Our Phase 2 clinical trial protocol for secretin in schizophrenia
            has been accepted by the FDA and we plan to initiate this study at 5
            centers in the United States later this month. This study will focus
            on the social deficits characteristic of many patients with
            schizophrenia and will target patients who are refractory to the
            current standard of care. There are approximately 2 million people
            in the United States with schizophrenia.

      |X|   Secretin was shown to inhibit anxiety in a well-recognized animal
            model of fear and anxiety. This study was published in
            Psychopharmacology in October. Previous studies have established
            that activity in this model is correlated with anxiolytic activity
            in patients. This study provides additional evidence that secretin
            modulates the activity of the amygdala, part of a neural network
            implicated in the acquisition and expression of emotional and social
            behaviors, including fear and anxiety. We are currently evaluating
            how best to investigate the use of secretin in the treatment of
            anxiety disorders and plan to initiate a clinical study in an
            anxiety disorder in 2004.

      |X|   We initiated a clinical trial in healthy volunteers to assess the
            safety, tolerability and pharmacokinetic profile of secretin when
            administered via subcutaneous injection. The goal of this study is
            to establish a dose of secretin when administered by subcutaneous
            injection that will be comparable to the blood levels that can be
            achieved when administered by intravenous injection. Subcutaneous
            dosing has the potential advantage of being administered in the home
            or in a community-based setting.

      |X|   Repligen and The University of Michigan are prosecuting a lawsuit
            against Bristol-Myers Squibb for correction of inventorship of
            certain CTLA4 patents issued to Bristol-Myers Squibb. In September,
            the District Court found that Repligen and the University of
            Michigan had not proven by clear, convincing and corroborative
            evidence that the University of Michigan scientist is a sole or
            joint inventor of any of the patents in suit. Repligen has filed a
            Notice of Appeal with the United States Court of Appeals for the
            Federal Circuit to the ruling of the United States District Court
            for the Eastern District of Michigan. We will continue to
            aggressively assert our position on the patents that are the subject
            of this lawsuit.

      |X|   Repligen received a Notice of Allowance from the United States
            Patent and Trademark Office for a patent covering the use of
            CTLA4-Ig for the treatment of rheumatoid arthritis, multiple
            sclerosis, systemic lupus erythematosis and scleroderma. This
            allowed patent will remain in force until 2020 and is independent
            from the patents on CTLA4-Ig which are the subject of the lawsuit
            that we are prosecuting against Bristol-Myers Squibb.


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Repligen Reports Second Quarter 2004 Financial Results, November 12, 2003
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About Repligen Corporation

Repligen Corporation is a biopharmaceutical company committed to being the
leader in the development of novel therapeutics for profound neuropsychiatric
disorders with particular emphasis on applications for children. Repligen has a
Specialty Pharmaceuticals business comprised of rProtein A(TM) and SecreFlo(TM),
the profits from which will be used to partially support the development of our
proprietary products. 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.

                             SELECTED FINANCIAL DATA

Operating Statement Data:



                                                  Three Months Ended                    Six Months Ended
                                                     September 30,                        September 30,
                                                2003              2002               2003               2002
                                           ---------------------------------------------------------------------
                                                                                        
Total revenue                              $  1,419,000       $  1,687,000       $  3,480,000       $  3,307,000
Cost of revenue                                 741,000            662,000          1,596,000          1,331,000
                                           ------------       ------------       ------------       ------------
Gross profit                                    678,000          1,025,000          1,884,000          1,976,000

Operating expenses:
  Research and development                    1,901,000          1,254,000          3,330,000          2,482,000
  Selling, general and administrative         1,011,000          1,010,000          2,914,000          1,892,000
                                           ------------       ------------       ------------       ------------
       Total costs and expenses               2,912,000          2,264,000          6,244,000          4,374,000
                                           ------------       ------------       ------------       ------------

Loss from operations
                                             (2,234,000)        (1,239,000)        (4,360,000)        (2,398,000)

Investment and interest income                   94,000            156,000            192,000            324,000
                                           ------------       ------------       ------------       ------------

Net loss                                   $ (2,140,000)      $ (1,083,000)      $ (4,168,000)      $ (2,074,000)
                                           ============       ============       ============       ============

Basic and diluted net loss per share
outstanding                                $       (.07)      $       (.04)      $       (.14)      $       (.08)
                                           ============       ============       ============       ============

Weighted average number of basic and
diluted shares outstanding                   29,860,000         26,643,000         29,424,000         26,643,000
                                           ============       ============       ============       ============



Balance Sheet Data:
                                September 30, 2003            March 31, 2003
                                ------------------            --------------

Cash and investments               $27,754,000                 $18,909,000

Total assets                        35,064,000                  26,793,000

Stockholders' equity                32,327,000                  24,550,000


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Repligen Reports Second Quarter 2004 Financial Results, November 12, 2003
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This release contains forward-looking statements which are made pursuant to the
safe harbor provisions of Section 27A of the Securities Act of 1933, as amended,
and Section 21E of the Securities Exchange Act of 1934, as amended. 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, plans and objectives for future operations, clinical
trials and results, litigation strategy, product research, 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.