A simple and extremely precise high-performance liquid chromatographic procedure has been developed for the determination of difloxacin and its metabolites in plasma and urine. Work-up of plasma samples entails ultrafiltration after addition of an internal standard in a displacing reagent containing sodium dodecyl sulfate. The ultrafiltrates are directly analyzed using a C18 reversed-phase analytical column, a soap-chromatographic mobile phase, and a fluorescence or ultraviolet detector. The mean intra-assay coefficient of variation for difloxacin over a concentration range of 10 ng/ml to 10 micrograms/ml was 0.5% when fluorescence detection and an internal standard were employed. Inter-assay coefficients of variation were approximately 2%. Recoveries of difloxacin and its metabolites were essentially quantitative and calibration curves were strictly rectilinear.