Oxacillin-resistant Staphylococcus aureus is rapidly killed by the endopeptidase lysostaphin, and the addition of beta-lactam antibiotics provides synergistic killing. We investigated the possibility that beta-lactams given in combination with lysostaphin would improve the activity of lysostaphin against oxacillin-resistant Staphylococcus epidermidis (ORSE), which is normally less susceptible to lysostaphin. Checkerboard synergy testing was performed for lysostaphin given in combination with oxacillin against 10 ORSE isolates for which the lysostaphin MICs were > o r= 8 microg/ml. The fractional inhibitory concentration index ranged from 0.0234 to 0.2656, indicating synergy, which was confirmed in growth curve experiments. In the rabbit model of experimental aortic valve endocarditis using an ORSE strain, the combination of lysostaphin and nafcillin was as effective as vancomycin alone and significantly better than lysostaphin or nafcillin alone. We conclude that beta-lactam antibiotics given in combination with lysostaphin are synergistic against many strains of ORSE.