Exhibit 99.1

Repligen Reports First Quarter 2021 Financial Results

and Updates Full Year 2021 Financial Guidance

WALTHAM, Mass. – May 4, 2021 — Repligen Corporation (NASDAQ:RGEN), a life sciences company focused on bioprocessing technology leadership, today reported financial results for its first quarter ended March 31, 2021. Provided in this press release are financial highlights for the quarter, updates to our financial guidance for the year 2021, and access information for today’s webcast and conference call.

Tony J. Hunt, President and Chief Executive Officer said, “I am extremely proud of the way our team executed here in the first quarter. With unprecedented demand driven by strong COVID tailwinds, our whole team pivoted to deliver an impressive quarter with 88% growth and 69% organic. COVID revenue contribution ticked up to 25% of revenues in the first quarter and equally impressive was the performance of our non-COVID base business which was up 31% year-on-year. This is an endorsement of the differentiation we have in the marketplace with our technologies and the focus and execution of our global team. With accelerating orders, new product launches, planned new capacity and impressive growth across all four franchises, we are confident about the full year outlook for the Company and are updating our financial guidance to reflect our expectations for operating margin expansion and for organic revenue growth in the range of 42%-49%.”

First Quarter 2021 Highlights

o COVID contributed 46 points, or 53% of revenue growth

o Base business (excluding COVID) contributed 27 points, or 31% of revenue growth

o Acquisitions made in 2020 contributed 14 points, or 16% of revenue growth

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Financial Details for the First Quarter 2021

REVENUE

GROSS PROFIT and GROSS MARGIN

OPERATING INCOME

NET INCOME

EARNINGS PER SHARE

EBITDA

CASH

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All reconciliations of GAAP to adjusted (non-GAAP) figures above, as well as EBITDA to adjusted EBITDA, are detailed in the reconciliation tables included later in this press release.

Financial Guidance for 2021

Our financial guidance for the fiscal year 2021 is based on expectations for our existing business and includes the financial impact of our 2020 acquisitions of ARTeSYN Biosolutions Holdings Ireland Limited (which closed on December 3, 2020), Non-Metallic Solutions, Inc. (which closed on October 20, 2020 ) and Engineered Molding Technologies LLC (which closed on July 13, 2020). The guidance below excludes the impact of potential additional acquisitions and future fluctuations in foreign currency exchange rates.

FISCAL YEAR 2021 GUIDANCE:

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Our non-GAAP guidance for the fiscal year 2021 excludes the following items:

Our non-GAAP guidance for the fiscal year 2021 includes:

All reconciliations of GAAP to adjusted (non-GAAP) guidance are detailed in the tables included later in this press release.

Conference Call

Repligen will host a conference call and webcast today, May 4, 2021, at 8:30 a.m. EST, to discuss first quarter financial results and corporate developments. The conference call will be accessible by dialing toll-free (844) 274-3999 for domestic callers or (412) 317-5607 for international callers. No passcode is required for the live call. In addition, a webcast will be accessible via the Investor Relations section of the Company’s website. Both the conference call and webcast will be archived for a period of time following the live event. The replay dial-in numbers are (877) 344-7529 from the U.S., (855) 669-9658 from Canada and (412) 317-0088 for international callers. Replay listeners must provide the passcode 10155233.

Non-GAAP Measures of Financial Performance

To supplement our financial statements, which are presented on the basis of U.S. generally accepted accounting principles (GAAP), the following non-GAAP measures of financial performance are included in this release: revenue growth rate at constant currency, adjusted gross profit and adjusted gross margin, adjusted income from operations and adjusted operating margin, earnings before interest, taxes, depreciation and amortization (EBITDA), adjusted EBITDA, adjusted net income, adjusted net income per share, adjusted earnings per diluted share (EPS), adjusted cost of sales, adjusted R&D expense, adjusted SG&A, adjusted income tax expense and adjusted income tax rate. The Company provides organic revenue growth rates in constant currency to exclude the impact of both foreign currency translation, and the impact of acquisition revenue for current year periods that have no prior year comparable, in order to facilitate a comparison of its current revenue performance to its past revenue performance. The Company provides revenue growth rates in constant currency in order to facilitate a comparison of its current revenue performance to its past revenue performance. To calculate revenue growth rates in constant currency, the Company converts actual net sales from local currency to U.S. dollars using constant foreign currency exchange rates in the current and prior period.

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The Company’s non-GAAP financial results and/or non-GAAP guidance exclude the impact of: acquisition and integration costs, inventory step-up charges and intangible amortization costs related to the Company’s acquisitions, as well as non-cash interest expenses and loss on conversion of debt related to the Company’s convertible debt, and the related impact on tax of non-GAAP charges. These costs are excluded because management believes that such expenses do not have a direct correlation to future business operations, nor do the resulting charges recorded accurately reflect the performance of our ongoing operations for the period in which such charges are recorded.

A reconciliation of GAAP to adjusted non-GAAP financial measures is included as an attachment to this press release. When analyzing the Company’s operating performance and guidance investors should not consider non-GAAP measures as substitutable for the comparable financial measures prepared in accordance with GAAP.

About Repligen Corporation

Repligen Corporation is a global life sciences company that develops and commercializes highly innovative bioprocessing technologies and systems that increase efficiencies in the process of manufacturing biological drugs. Our primary customers are biopharmaceutical drug developers and contract development and manufacturing organizations (CDMOs) worldwide. Our corporate headquarters are located in Waltham, Massachusetts, with additional administrative and manufacturing operations worldwide. The majority of our manufacturing sites are located within the U.S. (California, Massachusetts, New Jersey and New York), and outside of the U.S. we have sites in Estonia, Germany, Ireland, the Netherlands and Sweden.

The following constitutes a “Safe Harbor” statement under the Private Securities Litigation Reform Act of 1995: This press 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. Investors are cautioned that statements in this press release which are not strictly historical statements, including, without limitation, express or implied statements or guidance regarding current or future financial performance and position, including cash and investment position, demand in the markets in which we operate, the expected performance of our business, the expected performance of the Engineered Molding Technology, Non-Metallic Solutions and ARTeSYN Biosolutions businesses, the expected performance and success of our strategic partnerships, management’s strategy, plans and objectives for future operations or acquisitions, product development and sales, selling, general and administrative expenditures, intellectual property, development and manufacturing plans, availability of materials and product and adequacy of capital resources, our financing plans, and the projected impact of, and response to, the COVID-19 coronavirus pandemic on our business and on the U.S. and global economies constitute forward-looking statements identified by words like “believe,” “expect,” “may,” “will,” “should,” “seek,” “anticipate,” “projected,” “estimated,” “planned,” or “could” and similar expressions. Forward-looking statements are neither historical facts nor assurances of future performance. Because forward-looking statements relate to the future, they 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 following: the effect of the COVID-19 coronavirus pandemic, including mitigation efforts and economic effects, on our business operations and the operations of our customers and suppliers; the ultimate impact of the COVID-19 coronavirus pandemic on our business or financial results; our ability to successfully grow our bioprocessing business, including as a result of acquisition, commercialization or partnership opportunities; our ability to successfully integrate any acquisitions, our ability to develop and commercialize products and the market acceptance of our products; our ability to integrate the Engineered Molding Technology, Non-Metallic Solutions and ARTeSYN Biosolutions businesses successfully into our business and achieve the expected benefits of the acquisitions; reduced demand for our products that adversely

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impacts our future revenues, cash flows, results of operations and financial condition; our ability to compete with larger, better financed bioprocessing, pharmaceutical and biotechnology companies; our compliance with all U.S. Food and Drug Administration and EMEA regulations; our volatile stock price; and other risks detailed in Repligen’s Annual Report on Form 10-K for the year ended December 31, 2020 and the most recently filed Quarterly Report on Form-10-Q on file with the Securities and Exchange Commission and the other reports that Repligen periodically files with the Securities and Exchange Commission. Actual results may differ materially from those Repligen contemplated by these forward-looking statements. Therefore, you should not rely on any of these forward-looking statements. These forward-looking statements reflect management’s current views, expectations and assumptions regarding the future of our business, future plans and strategies, projections, anticipated events and trends, the economy and other future conditions, and is based on only on information currently available to us. Repligen does not undertake to update, whether written or oral, any of these forward-looking statements to reflect a change in its views or events or circumstances, whether as a result of new information, future development or otherwise, that occur after the date hereof except as required by law.

Repligen Contact:

Sondra S. Newman

Global Head of Investor Relations

(781) 419-1881

investors@repligen.com

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REPLIGEN CORPORATION

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS

(Unaudited, amounts in thousands, except share and per share data)

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REPLIGEN CORPORATION

RECONCILIATION OF GAAP INCOME FROM OPERATIONS TO

NON-GAAP (ADJUSTED) INCOME FROM OPERATIONS

(Unaudited, amounts in thousands)

REPLIGEN CORPORATION

RECONCILIATION OF GAAP NET INCOME TO NON-GAAP (ADJUSTED) NET INCOME

(Unaudited, amounts in thousands)

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REPLIGEN CORPORATION

RECONCILIATION OF GAAP NET INCOME PER SHARE TO

NON-GAAP (ADJUSTED) NET INCOME PER SHARE

(Unaudited)

REPLIGEN CORPORATION

RECONCILIATION OF GAAP NET INCOME TO ADJUSTED EBITDA

(Unaudited, amounts in thousands)

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REPLIGEN CORPORATION

RECONCILIATION OF GAAP COST OF SALES TO NON-GAAP (ADJUSTED) COST OF SALES

(Unaudited, amounts in thousands)

REPLIGEN CORPORATION

RECONCILIATION OF GAAP R&D EXPENSE TO NON-GAAP (ADJUSTED) R&D EXPENSE

(Unaudited, amounts in thousands)

REPLIGEN CORPORATION

RECONCILIATION OF GAAP SG&A EXPENSE TO NON-GAAP (ADJUSTED) SG&A EXPENSE

(Unaudited, amounts in thousands)

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REPLIGEN CORPORATION

RECONCILIATION OF GAAP NET INCOME GUIDANCE TO ADJUSTED (NON-GAAP NET INCOME GUIDANCE)

REPLIGEN CORPORATION

RECONCILIATION OF GAAP NET INCOME PER SHARE GUIDANCE TO

ADJUSTED (NON-GAAP) NET INCOME PER SHARE GUIDANCE

# # #

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