Human blood phagocytes and opsonized Staphylococcus aureus were incubated together in vitro and then treated with the bacteriolytic enzyme lysostaphin in order to remove extracellular bacteria. After washing, the phagocytic cells were incubated for a further 90 min with or without added antibiotics and the effect of these drugs on the survival of sensitive intracellular Staph. aureus measured. Penicillin and cefazolin had no effect on intracellular staphylococcal survival, but rifampicin, streptomycin and gentamicin reduced intracellular bacterial counts. The intracellular survival of antibiotic-resistant strains of Staph. aureus was not affected by the relevant extracellular antibiotics. Pre-incubation of phagocytic cells with either aminoglycosides or rifampicin had no effect on the intracellular survival of antibiotic-sensitive Staph. aureus.