Initial clinical trials with daptomycin (2 mg/kg per day) were prematurely suspended because of unexplained treatment failures in patients with bacteremia who were treated with daptomycin, despite in vitro data indicating that the gram-positive cocci causing the infection were susceptible to daptomycin. One explanation for these clinical failues may relate to the relatively high degree of daptomycin protein binding (94%). To evaluate the impact of protein on daptomycin activity, a two-chamber in vitro pharmacodynamic model was used to study and compare the interaction between Staphylococcus aureus (clinical isolate) and either daptomycin or vancomycin, each in the presence and absence of physiologic human albumin concentrations. Low-dose (2 mg/kg) daptomycin, high-dose (6 mg/kg) daptomycin, and 10 mg of vancomycin per kg beta-phase elimination serum-concentration-versus-time curves were simulated by using this in vitro pharmacodynamic model. The bacterial kill rates by all three regimens were decreased in the presence of albumin (P < 0.0002). The average times required for a 99% kill of the initial S. aureus inocula (approximately 5 x 107 CFU/ml) without albumin were 0.81 (low-dose daptomycin), 0.33 (high-dose daptomycin), and 6.18 (vancomycin) h. The average times required for a 99% kill of S. aureus with albumin were 7.66 (low-dose daptomycin), 0.95 (high-dose daptomycin), and 10.52 (vancomycin) h. These data demonstrate that, depending on the concentration of daptomycin, the presence of albumin can profoundly diminish the bactericidal activity of daptomycin.