Phage-coded lysin is an enzyme that destroys the cell walls of bacteria. Phage lysin could be an alternative to conventional antibiotic therapy against pathogens that are resistant to multiple antibiotics. In this study, a novel staphylococcal phage, GH15, was isolated, and the endogenous lytic enzyme (LysGH15) was expressed and purified. The lysin LysGH15 displayed a broad lytic spectrum; in vitro treatment killed a number of Staphylococcus aureus strains rapidly and completely, including methicillin-resistant S. aureus (MRSA). In animal experiments, a single intraperitoneal injection of LysGH15 (50 μg) administered 1 h after MRSA injections at double the minimum lethal dose was sufficient to protect mice (P < 0.01). Bacteremia in unprotected mice reached colony counts of about 10(7) CFU/ml within 3.5 h after challenge, whereas the mean colony count in lysin-protected mice was less than 10(4) CFU/ml (and ultimately became undetectable). These results indicate that LysGH15 can kill S. aureus in vitro and can protect mice efficiently from bacteremia in vivo. The phage lysin LysGH15 might be an alternative treatment strategy for infections caused by MRSA.