In order to determine the microbiological and pharmacokinetic parameters that best predicted the in vivo antistaphylococcal activity of the streptogramin RP 59500 (quinupristin-dalfopristin), we evaluated the activity in rabbit aortic endocarditis of three regimens of quinupristin-dalfopristin against five strains of Staphylococcus aureus with various streptogramin B-type antibiotic resistance phenotypes and susceptible to streptogramin A-type antibiotics. Quinupristin-dalfopristin was as active as vancomycin against three strains that were susceptible to its streptogramin B component quinupristin, including one strain that was inducibly resistant to erythromycin, but had a significantly decreased activity against two strains that were resistant to quinupristin, for all quinupristin-dalfopristin regimens tested (P < 0.05). The area under the concentration-time curve for quinupristin-dalfopristin in plasma divided by the MIC of quinupristin was the only parameter retained by multilinear regression that predicted the in vivo activity of quinupristin-dalfopristin (P = 0.0001), emphasizing the importance of determining the susceptibility to quinupristin in order to predict the in vivo activity of quinupristin-dalfopristin against S. aureus.