May 1. 2003

FOR IMMEDIATE RELEASE

RIB-X PHARMACEUTICALS COMPLETES $63.5 MILLION
SERIES B FINANCING

New Haven, CT, May 1, 2003 -- Rib-X Pharmaceuticals, Inc., a privately held biotechnology company focused on the discovery of novel anti-infectives, announced today the completion of a $63.5 million Series B Preferred Stock financing, in a round led by Warburg Pincus, a leading private equity investor in healthcare. This brings the total amount raised by the Company, since its inception in 2000, to more than $73.0 million.

Warburg Pincus, a new investor, led the Series B Financing with a $31.0 million investment. All of the Company’s previous investors also participated in this round; they include: Axiom Venture Partners, Cardinal Partners, Connecticut Innovations, EuclidSR Partners, Oxford Bioscience Partners, S.R. One, Limited and Zero Stage Capital.

"We are proud to have merited this level of investor confidence from such a distinguished group of investors, particularly given current market conditions, and believe it demonstrates the potential of our technology and our success in implementing our business plan," said Dr. Susan Froshauer, President and Chief Executive Officer of Rib-X. "We thank our existing shareholders, as well as Warburg Pincus for their support."

The financing consisted of $51.0 million of new funding received today from new and existing investors, in addition to the receipt of $12.5 million committed capital from investors who participated in Rib-X’s earlier venture round.

"Utilizing less than $10 million from our initial fund raising and in a year and one-half, we have established the Company, assembled a top-tier, multifaceted research and development team and initiated several parallel candidate generation programs,” added Dr. Froshauer. “We are confident that the infusion of the $63.5 million will power our discovery and development efforts to pursue several series of compounds into Phase II clinical trials.”

In conjunction with the financing, the Company also announced that Stewart Hen and Jonathan Leff, both Managing Directors of Warburg Pincus will be joining the Rib-X Board of Directors effective immediately, bringing the total number of board members to seven.

"Rib-X has made tremendous progress since it began its discovery programs approximately eighteen months ago,” said Mr. Hen. “We believe the Company’s technology platform has the potential to generate fundamentally new classes of antibiotics that cannot be developed using traditional drug discovery approaches. If successful, these compounds will provide important new weapons for physicians to address the rapidly growing problem of resistant organisms in both the hospital and community settings. We look forward to working with Rib-X to maximize the Company’s potential.”

About Rib-X
Rib-X Pharmaceuticals is a small molecule drug discovery company focused on the structure-based design of new classes of anti-infective agents. The Company’s drug discovery strategy is focused on the exploitation of its proprietary high resolution crystal structure of the 50S subunit of the ribosome to which many known antibiotics bind. The Company’s integrated research approach combines state of the art, proprietary computational analysis, x-ray crystallography, medicinal chemistry, microbiology and biochemistry, allowing for the rapid synthesis of new agents which avoid typical antibiotic resistance mechanisms. Utilizing these tools, Rib-X is optimizing leads to create new classes of antibiotics.

Contacts
Rx Communications Group
Rhonda Chiger (investors)
917-322-2569
rchiger@rxir.com

Pete Holmberg (media)
917-322-2164
pholmberg@rxir.com

About Warburg Pincus
Warburg Pincus is a global private equity firm and a leading investor in healthcare, biotechnology and specialty pharmaceuticals. Since 1971, Warburg Pincus has sponsored ten private equity investment funds with committed capital in excess of $19 billion. The firm is currently investing its eighth global fund, Warburg Pincus Private Equity VIII, which had a final closing in April 2002 at $5.3 billion. Warburg Pincus has invested more than $800 million in the biotechnology industry and helped to build more than 40 biotechnology and biopharmaceutical companies spanning a broad range of technologies and stages of development. For more information, please visit our website at www.warburgpincus.com.