The in-vitro activity of RP 59500 and comparative agents was determined against 270 clinical isolates of the genus Staphylococcus. MICs were performed by microdilution dilution. MIC 90 , and MBC 90 (mg/L) of RP 59500 were as follows: oxacillin-sensitive Staphylococcus aureus (0·5/0·5), oxacillin-resistant S. aureus (0·5/0·5), oxacillin-sensitive Staphylococcus epidermidis (0·5/0·5), oxacillin-resistant S. epidermidis (0·25/0·25), oxacillin-resistant Staphylococcus hominis (1·0/1·0), oxacillin-resistant Staphylococcus haemolyticus (1·0/1·0), oxacillin-sensitive Staphylococcus saprophyticus (10/1·0). Killing kinetic methods were used to assess the bactericidal activity of inhibitory (1 ×, 2 × MIC) concentrations of RP 59500 in comparison with that of vancomycin (1 ×, 2 × MIC) and oxacillin (1 ×, 2 × MIC) against 20 strains of oxacillin-sensitive and -resistant S. aureus and S. epidermidis . RP 59500 was as active as vancomycin, displaying rapid bactericidal activity against the majority of strains tested and reducing initial inoculum counts by > 99·9% in 2–12 h. Regrowth was seen with some S. epidermidis strains after 12–24 h.