Killing kinetic experiments were performed with ciprofloxacin, fusidic acid and rifampicin alone and with ciprofloxacin combined with either fusidic acid or rifampicin against ten strains of Staphylococcus aureus. The strains were all clinical isolates and two strains were methicillin-resistant. The antibiotic concentrations tested were in the range obtainable in the serum with recommended doses. The early bactericidal effect of ciprofloxacin alone was substantially greater than that of fusidic add or rifampicin; thus the extent of killing after 6 h exposure to these antibiotics was 3.4 log 10, 0.8 log10 and 0.6 log10, respectively. Fusidic acid as well as rifampicin antagonized the bactericidal activity of ciprofloxacin. Each combination killed 2 log10 cfu less than ciprofloxacin alone. For each combination the mean decrease in the number of organisms was comparable to that achieved with fusidic add or rifampicin used alone.