The pharmacokinetic properties of rufloxacin, a new quinolone antibacterial agent, were evaluated in ten patients with lower respiratory tract infections. Patients were given 400 mg of rufloxacin once a day for seven to nine days. Plasma concentrations of the drug were determined by high-performance liquid chromatography and bioassay at regular intervals during treatment. After the first administration, maximal plasma concentrations were 3.17 +/- 0.36 mg/l (mean +/- SEM) and were reached at 4.2 +/- 0.7 h. At the end of treatment peak plasma concentrations increased to 7.26 +/- 0.52 mg/l. Elimination half-life was 38.2 +/- 2.9 h, with a mean extent of accumulation of 2.96 +/- 0.30. Treatment was well tolerated, with no abnormalities noted during routine laboratory examinations. Two days after the last administration, measurable levels of rufloxacin were still observed in plasma, indicating that the long half-life of rufloxacin assures valuable antibacterial activity even after discontinuation of treatment.