The serum bactericidal activity of ciprofloxacin against strains of Streptococcus pneumoniae, Staphylococcus aureus, and Pseudomonas aeruginosa for which MICs are similar (0.4 microgram/ml) was assessed with serum ultrafiltrates from five healthy volunteers receiving ciprofloxacin at 400 mg intravenously every 8 h. In addition, human serum was supplemented with ciprofloxacin to achieve a mean steady-state concentration (Css) that might be achieved in patients with renal failure, with total clearances of 3 to 4 liters/h (elimination rate constant, 0.08 h-1). The area under the inhibitory titer curve from 0 to 24 h (AUIC24) and the area under the bactericidal titer curve from 0 to 24 h (AUBC24) were both measured and predicted as the area under the concentration-time curve from 0 to 24 h (AUC24)/MIC and AUC24/MBC, respectively. We previously demonstrated that a breakpoint AUC24/MIC of 125 for ciprofloxacin had a significantly higher probability of treatment success than lower values, with 250 to 500 being optimal. Volunteer sera (mean Css, 1.55 to 2.48 micrograms/ml) achieved AUC24/MICs of 90 to 145. Supplemented serum (mean Css, 6.00 to 7.42 micrograms/ml) achieved AUC24/MICs of 350 to 450. Correlation coefficients for measured and predicted values of AUC24/MIC and AUC24/MBC were 0.826 and 0.941, respectively. The mean percent errors were not significantly different from zero for either AUIC24 or AUBC24 values (P > 0.1, P > 0.4). Time-kill curve studies were performed with low (1.55 to 2.48 micrograms/ml), intermediate (6.00 to 7.42 micrograms/ml), and high (15 to 25 micrograms/ml) concentrations of ciprofloxacin for the three organisms. At low concentrations (3 to 6 times the MIC) AUC24/MICs were <125 for two of five volunteers and the killing rates were considerably more rapid for P. aeruginosa than for S. pneumoniae or S.aureus. Intermediate concentrations (15 to 18 times the MIC) achieved optimal AUC24/MICs, and the killing rates were similar for the three organisms. A paradoxical decrease in the killing rate was seen at high concentrations (35 to 60 times the MIC). At clinically achievable concentrations, ciprofloxacin killed P. aeruginosa more rapidly than it did either S. pneumoniae or S. aureus.