The in vitro antistaphylococcal activity of RP 59500, a new streptogramin, was comparable to those of vancomycin and teicoplanin against Staphylococcus aureus, and RP 59500 was the most active agent against coagulase-negative staphylococci. All staphylococcal strains were inhibited by 4 micrograms of RP 59500 per ml, including multiply resistant strains. Broth microdilution and agar dilution testing gave comparable results in 97% of the tests with RP 59500.