Although conventional amphotericin B was for many years the drug of choice and remains an important agent against invasive aspergillosis, reliable susceptibility breakpoints are lacking. Three clinical Aspergillus isolates (Aspergillus fumigatus, Aspergillus flavus, and Aspergillus terreus) were tested in an in vitro pharmacokinetic-pharmacodynamic model simulating the biphasic 24-h time-concentration profile of free amphotericin B concentrations in human serum with free peak concentrations (fC(max)) of 0.1, 0.3, 0.6, 1.2, and 2.4 mg/liter administered once daily. Drug concentrations were measured with a bioassay, and fungal growth was monitored for 72 h with galactomannan production. The fC(max)/MIC corresponding to half-maximal activity (P-50) was determined for each species, and the percentage of target attainment was calculated for different MICs for the standard (1 mg/kg of body weight) and a lower (0.6-mg/kg) dose of amphotericin B with Monte Carlo simulation analysis. The fC(max)/MICs (95% confidence intervals) corresponding to P-50 were 0.145 (0.133 to 0.158), 0.371 (0.283 to 0.486), and 0.41 (0.292 to 0.522) for A. fumigatus, A. flavus, and A. terreus, respectively. The median percentages of P50 attainment were >= 88%, 47%, and 0% for A. fumigatus isolates with MICs of <= 0.5, 1, and >= 2 mg/liter, respectively, and >= 81%, 24%, and 0% and >= 75%, 15%, and 0% for A. flavus and A. terreus isolates with MICs of <= 0.25, 0.5, and >= 1 mg/liter, respectively. The lower dose of 0.6 mg/kg would retain efficacy for A. fumigatus, A. flavus, and A. terreus isolates with MICs of <= 0.25, <= 0.125, and <= 0.125 mg/liter, respectively. The susceptibility, intermediate susceptibility, and resistance breakpoints of <= 0.5, 1, and >= 2 mg/liter for A. fumigatus and <= 0.25, 0.5, and >= 1 mg/liter for A. flavus and A. terreus were determined for conventional amphotericin B with a pharmacokinetic-pharmacodynamic model simulating free-drug serum concentrations.