OBJECTIVE: To assess the antibacterial action of moxifloxacin on Haemophilus influenzae and Moraxella catarrhalis using an in vitro pharmacodynamic model of infection. METHODS: Serum concentrations in humans associated with doses of 400 mg once a day for 48 h were simulated and the antibacterial effect measured by the log change in viable count at intervals through the simulation compared to time zero and also the area under the bacterial kill curve (AUBKC). Wild-type strains of H. influenzae and M. catarrhalis with varying beta-lactam susceptibilities (moxifloxacin MICs </=0.1 mg/L) and laboratory-generated mutants with moxifloxacin MICs of 0.5-1.0 mg/L were tested. RESULTS: H. influenzae strains with MICs of </=0.06 mg/L were eradicated by 12 h; strains with MICs of 0.5-1.0 mg/L were eradicated by 24-36 h. M. catarrhalis strains with MICs of </=0.08 mg/L were eradicated by 24-36 h, while the less susceptible laboratory mutant (MIC 1.0 mg/L) was not eradicated even at 48 h. Although we used only a single dosing simulation, the AUC/MIC ratio was strongly related to the AUBKC. CONCLUSIONS: These data indicated that once daily dosing of moxifloxacin holds promise for the treatment of infection due to H. influenzae or M. catarrhalis but bacterial killing is related to MIC.