The bactericidal activity of rifampin plus other antibiotics was examined with in vitro checkerboard dilutions against 15 strains of staphylococci, with time-kill studies, and with determinations of serum bactericidal activities (SBAs) in volunteers. In vitro synergism was found consistently only in inhibitory assays with rifampin plus minocycline. Time-kill studies with rifampin plus methicillin revealed enhanced killing at concentrations close to the MBC but reduced combined activity at higher concentrations. SBAs with rifampin, oxacillin, and the two drugs in combination were studied in three treatment regimens (regimen A: 2 g of oxacillin and 300 mg of rifampin; regimen B: 1 g of oxacillin and 450 mg of rifampin; and regimen C: 3 g of oxacillin and 450 mg of rifampin). The SBA against methicillin-sensitive Staphylococcus aureus was significantly lower with the two drugs in combination than with oxacillin alone in regimen A but not in regimen B. In vitro synergy with rifampin plus other antibiotics was not frequent. At high concentrations, the bactericidal activity of oxacillin was reduced by rifampin, but killing activity may be enhanced at low concentrations of oxacillin.