Clinafloxacin (CI-960) is a potent broad-spectrum, fluoroquinolone antibiotic that has been studied for parenteral and oral administration in patients with serious infections. The objectives of these studies were to examine the pharmacokinetics and safety of clinafloxacin following administration of single and twice-daily intravenous (i.v.) and oral doses to volunteers. Plasma and urine samples were assayed by validated liquid chromatographic methods, and pharmacokinetic parameter values were determined by noncompartmental methods. Safety was evaluated by clinical observation and laboratory tests. Absorption was rapid after oral administration, with maximum concentrations in plasma (C_max) generally occurring within 2 h. Concentrations in plasma declined biexponentially, with an average terminal half-life of 4 to 6 h after single doses and 5 to 7 h after multiple doses. Increases in C_max and area under the concentration-time curves (AUC) were generally proportional to the dose. The volume of distribution was much greater than total body water. Approximately 40 to 75% of the clinafloxacin doses were excreted unchanged into urine. Absolute bioavailability of orally administered clinafloxacin was approximately 90% and did not change with increasing dose. Therefore, switching patients from i.v. to oral dosing should achieve similar concentrations in plasma. The tolerability of clinafloxacin was acceptable. No serious adverse events occurred. C_max values and minimum plasma clinafloxacin concentrations during multiple dosing exceeded MICs for a wide range of organisms.