In a randomised crossover study, 14 volunteers received a single oral dose of 500 mg levofloxacin or 500 mg ciprofloxacin in order to assess plasma concentrations by high-pressure liquid chromatography (up to 24 h), urinary excretion and urinary bactericidal titres (UBTs) at intervals up to 120 h. The median maximum concentration of levofloxacin in plasma was 6.1 mg/L and that of ciprofloxacin was 2.3 mg/L. The median cumulative level of renal excretion of the administered dose of the parent drug was 81.2% for levofloxacin and 36.2% for ciprofloxacin. UBTs were determined for a reference strain and nine clinical uropathogens. The median UBTs of both quinolones measured within the first 12h were between 0 and 1:> or =1024, correlating with the minimum inhibitory concentrations (MICs) of the strains. For Gram-negative strains, the UBTs of both quinolones were comparable despite the lower MICs of ciprofloxacin. During further time courses, however, the UBTs of levofloxacin were significantly higher than those of ciprofloxacin. For Gram-positive strains, for which the MICs of levofloxacin were equal to or lower than those of ciprofloxacin, the UBTs of levofloxacin were already significantly higher from the beginning. It can be concluded that overall the doses of the two tested fluoroquinolones may be considered equivalent with regard to treatment of complicated urinary tract infections, although the recommended dosing is twice daily for ciprofloxacin and once daily for levofloxacin.