Because rifampin-resistant strains of Staphylococcus aureus emerge during monotherapy with this drug, a search was made for potentially useful companion drugs. Bactericidal titers of spiked serum were determined, and time kill studies were performed for 10 strains of methicillin-susceptible S. aureus. We tested rifampin in combination with nafcillin, vancomycin, clindamycin, pefloxacin, ciprofloxacin, trimethoprim, teicoplanin, or erythromycin. The bactericidal activity of nafcillin, vancomycin, and teicoplanin was significantly reduced (P less than 0.05) when rifampin was added to the drug regimen. In contrast, the addition of rifampin to clindamycin or erythromycin significantly increased bactericidal activity as measured by both bactericidal titers in serum and 6-h killing rates (P less than 0.02). Bactericidal activity in serum was also increased by the addition of rifampin to trimethoprim, but rifampin-resistant strains emerged with this combination.