Cubist Pharmaceuticals Announces Significant Sales Milestone; CUBICIN Fastest to Reach $100 Million Net Product Revenues Since Launch for an IV Antibiotic in U.S.; Almost 85,000 Patients Treated to Date for Serious Infections.

LEXINGTON, Mass. -- Cubist Pharmaceuticals, Inc. (Nasdaq: CBST) today announced that, earlier this month, its first-in-class IV antibiotic, CUBICIN(R) (daptomycin for injection), passed $100 million in cumulative net product revenues since its launch in November, 2003. CUBICIN has reached this sales milestone faster than any IV antibiotic ever launched in the U.S. In addition to announcing the sales milestone, the Company reiterated its previously announced CUBICIN revenue guidance for 2005 of $110-120 million.

Mike Bonney, Cubist President and CEO, said, "I give great credit to the employees of Cubist, and, in particular, to our Commercial Operations, Manufacturing and Quality groups, for the success of the CUBICIN launch. CUBICIN has now been used in the treatment of almost 85,000 patients with serious infections. This is something everyone at Cubist feels proud of. We eagerly await the additional clinical data we will share in the middle of the year when results are available from the CUBICIN Phase 3 trial for treatment of infective endocarditis/bacteremia caused by Staphylococccus aureus."

About CUBICIN(R) (daptomycin for injection)

CUBICIN is currently the only once-daily bactericidal antibiotic approved in the U.S. indicated for the treatment of complicated skin and skin structure infections caused by susceptible strains of the following Gram-positive microorganisms: Staphylococcus aureus (including methicillin-resistant strains), Streptococcus pyogenes, S. agalactiae, S. dysgalactiae subsp equisimilis and Enterococcus faecalis (vancomycin-susceptible strains only). CUBICIN is not indicated for the treatment of pneumonia. Most adverse events reported in clinical trials were mild or moderate in intensity and the most common were constipation, nausea, injection site reactions, and headache. To reduce the development of drug-resistant bacteria and maintain the effectiveness of CUBICIN, CUBICIN should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria susceptible to CUBICIN. For full prescribing information, visit www.Cubicin.com.

About Cubist

Cubist Pharmaceuticals, Inc. is a biopharmaceutical company focused on the research, development and commercialization of antiinfective products that meet unmet medical needs. In the U.S., Cubist markets CUBICIN(R) (daptomycin for injection), the first antibiotic in a new class of antiinfectives called lipopeptides. Cubist's pipeline includes HepeX-B(TM), a monoclonal antibody biologic currently in the second of two Phase 2 trials to determine its potential for the prevention of infection by the Hepatitis B virus (HBV) in liver transplant patients, and research efforts focused on novel members of the lipopeptide class of molecules and on natural products discovery. Cubist is headquartered in Lexington, MA.

Cubist Safe Harbor Statement

Statements contained herein that are not historical fact may be forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, and such statements are subject to a variety of risks and uncertainties. There are a number of important factors that could cause actual results to differ materially from those projected or suggested in any forward-looking statements made by Cubist. These factors include, but are not limited to: (i) the level of acceptance of CUBICIN by physicians, patients, third-party payors, and the medical community generally; (ii) Cubist's ability to continue to develop, secure additional regulatory approvals for, and successfully market, CUBICIN; (iii) Cubist's expectations regarding our ability to continue to manufacture sufficient quantities of CUBICIN in accordance with current Good Manufacturing Practices; (iv) commercialization of products that are competitive with CUBICIN; (v) Cubist's ability to discover or in-license drug candidates; (vi) Cubist's ability to successfully develop drug candidates in its pipeline, including HepeX-B; (vii) Cubist's ability to successfully commercialize any product or technology developed by Cubist; (viii) Cubist's ability to establish and maintain successful manufacturing, sales and marketing, distribution, and development collaborations; (ix) legislative or regulatory changes adversely affecting Cubist or the biopharmaceutical industry; (x) Cubist's expectations regarding the future market demand and medical need for CUBICIN; (xi) Cubist's ability to protect its intellectual property and proprietary technologies; and (xii) Cubist's ability to finance its operations. Additional factors that could cause actual results to differ materially from those projected or suggested in any forward-looking statements are contained in Cubist's recent filings with the Securities and Exchange Commission, including those factors discussed under the caption "Risk Factors" in such filings.

Cubist and CUBICIN are registered trademarks of Cubist Pharmaceuticals, Inc.; HepeX-B is a trademark of XTL Biopharmaceuticals Ltd.

Additional information can be found at Cubist's web site at www.cubist.com