A simple, rapid, and accurate high-performance liquid chromatographic method using fluorescence detection is described for the measurement of ciprofloxacin in plasma, whole blood, and erythrocytes. Ciprofloxacin and the internal standard difloxacin were separated on a mu-Bondapak C18 column (30 cm x 3.9 mm inside diameter, 10 microns particle size), using a mobile phase of 0.1 M phosphate buffer (pH 2.5):acetonitrile (75:25, vol/vol). The retention times were 5.1 min for ciprofloxacin and 7.9 min for difloxacin. The compounds were extracted from the three biological fluids using protein precipitation followed by a single-step liquid-liquid extraction. The assay is precise, with interassay coefficients of variation of less than or equal to 9.1% and an accuracy of less than or equal to 7.4% at 0.5 and 5.0 micrograms/ml (n = 5). The mean extraction recoveries of ciprofloxacin in plasma, whole blood, and erythrocytes were 84.4, 63.9, and 48.0%. The limit of detection for ciprofloxacin is 25 ng/ml. Ciprofloxacin concentrations in the three biological fluids were measured in patients with uncomplicated falciparum malaria to demonstrate the application of the method.