To compare the antimicrobial effects of clinically achievable ratios of area under the curve (AUC) to MIC, a clinical isolate of <i>Moraxella catarrhalis</i> was selected with MICs corresponding to the MIC₅₀s of four quinolones. Monoexponentially declining concentrations observed in human plasma after oral administration of 1,000 mg of ciprofloxacin (as two 500-mg doses at a 12-hour interval), 320 mg gemifloxacin, 500 mg levofloxacin or 400 mg moxifloxacin (each as a single dose) and were simulated in an in vitro dynamic model. The respective half-lives were 4, 7.4, 6.8 and 12.1 h, and the AUC/MICs were 730, 1,130, 920 and 690 h. The time-kill/regrowth curves yielded similar patterns with the four quinolones: a rapid reduction in bacterial numbers followed by bacterial regrowth that occurred later with moxifloxacin than with ciprofloxacin, gemifloxacin, and levofloxacin. The total antimicrobial effect of moxifloxacin as expressed by the <i>I</i>_E parameter (area between the control growth and time- kill curves from time zero to the time when bacterial counts on the regrowth curve achieve the same maximal numbers as in the absence of antimicrobial) was 30, 55, and 120% greater than gemifloxacin, levofloxacin and ciprofloxacin, respectively. Unlike <i>I</i>_E, the other integral indices determined over a fixed time (24 h) – the area between the control growth and time-kill curves, area above the time-kill curve and area under the time-kill curve were similar for the four fluoroquinolones, thus precluding their differentiation.