Ciprofloxacin was given orally to 10 healthy volunteers for seven consecutive doses of 250 mg every 12 h. Serum and urine samples were collected at distinct times between 0 and 96 h and analyzed both by high-pressure liquid chromatography and by a microbiological assay. The detection limits were 0.006 and 0.03 microgram/ml, respectively. For each method, imprecision coefficients of variation were less than 6.1% at various concentrations in serum and urine. The means +/- standard deviations of the absolute values of the relative differences between the two methods were 9.3 +/- 6.8% (n = 225) for serum samples and 58.5 +/- 50.4% (n = 70) for urine samples. Comparison of the concentrations in serum measured with high-pressure liquid chromatography and bioassay by regression analysis yielded a slope which was not significantly different from 1.0 (99.9% confidence limits: 0.984 less than slope less than 1.035). In urine, however, the bioassay results were markedly higher than the high-pressure liquid chromatography values (1.327 less than slope less than 1.698), which indicates the presence of antimicrobially active metabolites. The cumulative 12-h urinary recovery after the first and seventh doses averaged 30.2 +/- 8.5 and 26.4 +/- 4.6%, respectively, by high-pressure liquid chromatography, whereas with bioassay 38.2 +/- 5.9 and 45.5 +/- 5.9% activity was recovered. Protein binding appeared to be neither concentration nor pH dependent and averaged 21.9 +/- 4.1%.