The effect of rifampicin in combination with dicloxacillin or fusidic acid on the extracellular and intracellular killing of Staphylococcus aureus in human neutrophil granulocytes in the presence of serum was studied. At the extracellular level rifampicin significantly reduced the bactericidal activity of dicloxacillin, but had an indifferent effect on the activity of fusidic acid. The combination of rifampicin with dicloxacillin or fusidic acid led to intracellular killing no different from that produced by rifampicin alone. However, owing to the high intracellular activity of rifampicin, the intracellular killing by the drug combinations was greater than that by dicloxacillin or fusidic acid alone.