Vancomycin was used alone and in combination with rifampin in the treatment of experimental osteomyelitis due to Staphylococcus aureus in rabbits. Treatment with 60 mg of vancomycin/kg of body weight twice a day for 28 days was ineffective in sterilizing infected rabbit bones. Rifampin (40 mg/kg) injected once a day for 28 days sterilized 57% of infected rabbit bones. Treatment with a combination of vancomycin and rifampin for either 14 or 28 days was significantly more effective than either drug used alone, sterilizing 84% and 90%, respectively, of the infected bones of treated animals. A possible explanation for the failure of vancomycin when used alone may be that its in vitro activity against the infecting strain of S. aureus (as measured by minimal inhibitory concentrations or minimal bactericidal concentrations) was substantially less under anaerobic conditions (that is, at partial pressures of oxygen analogous to those in osteomyelitic bones) than under aerobic conditions.