Neither clindamycin nor lincomycin killed intracellular Staphylococcus aureus over a 4-h period. Bio-assay of neutrophil sonicates after exposure to antibiotic showed the presence of active clindamycin at approximately 20 times the extracellular concentration. Clindamycin and lincomycin kill staphylococci relatively slowly, particularly in cell culture medium and balanced salt solution. This might account for their failure to kill intracellular staphylococci despite intracellular accumulation. The neutrophil experiments could not be extended to a time period (20 h) over which the antibiotics would kill S. aureus, as the presence of these bacteria within neutrophils for this length of time resulted in considerable cell lysis.