Two column liquid chromatographic (HPLC) methods for the determination of ciprofloxacin and three metabolites are described. Both use reversed phase chromatography, the stationary phase being Nucleosil 5C18. Method A separates ciprofloxacin, metabolite M1 and another metabolite of unknown structure using fluorometric detection. Method B allows the determinations of metabolite M3 (oxo-ciprofloxacin) in urine by UV absorption. Serum was deproteinised with acetonitrile. Urine was diluted with buffer solution. The detection limit of ciprofloxacin was 0.010 mg/l serum and 0.2 mg/l urine and for the metabolite M3, 1 mg/l urine. Within-batch precision (coefficient of variation) for ciprofloxacin in serum was 0.8 to 2.4% and between-batch precision 4.8 to 9.3%. In urine within-batch precision was 1.7 to 2.1% and between-batch precision 2.4 to 7.2%. Recovery rates of ciprofloxacin from three groups of spiked sera was 94.5 +/- 2.6%, 97.2 +/- 1.1% and 95.0 +/- 1.8% and from urine 99.6%. Results obtained by HPLC (method A) were compared with those from a standard microbiological assay by means of bivariate regression analysis. In 12 subsets of data the slope of the regression line varied from 1.042 to 1.556. Significantly higher results from the microbiological assay were probably due to the presence of microbiologically active metabolites. We conclude that HPLC is the more specific method of determination. The described methods were applied for pharmacokinetic studies and therapeutic drug monitoring.