Macrophages obtained by culturing human blood monocytes were incubated with Staphylococcus aureus for phagocytosis to occur and exposed to gentamicin, rifampin, clindamycin or trimethoprim/sulphamethoxazole. The macrophage-associated bacteria were protected against gentamicin at low concentrations (1 mg/l) and trimethoprim/sulphamethoxazole. However, high concentrations of gentamicin and clindamycin reduced the number of bacteria, indicating that these drugs penetrated into human macrophages and killed phagocytosed bacteria. Rifampin, even at low concentrations (0.5 mg/l), caused a marked reduction in macrophage-associated bacteria, implying that the drug penetrated into the phagocytes and retained its effect in the cells most effectively.