A sensitive, specific and rapid liquid chromatographic procedure to monitor rufloxacin and theophylline selectively in human plasma has been developed and validated. Plasma samples were extracted with a mixture of dichloromethane–diethyl ether. The organic layer was evaporated to dryness, reconstituted with mobile phase, and the analytes were separated and quantified by LC on an octadecylsilane column with acetonitrile–0.1 mol dm^–3 phosphate buffer (pH 3) as mobile phase and UV detection at 280 nm. The procedure was linear from 0.05 to 5.0 µg cm^–3, with a detection limit of 0.03 µg cm^–3 for rufloxacin, and from 0.5 to 30 µg cm^–3, with a detection limit of 0.02 µg cm^–3 for theophylline. The accuracy of the method was defined by relative errors of 4.2, 5.0 and 1.6% for human plasma containing 0.1, 1.0 and 5.0 µg cm^–3 of rufloxacin and 2.4, 2.0 and 2.7% at the concentrations of 1.0, 5.0 and 20 µg cm^–3 for theophylline. The corresponding relative standard deviations were 3.4, 2.8 and 1.7% for rufloxacin and 2.1, 3.7 and 3.8% for theophylline at the same concentrations, respectively. The assay has been successfully applied to pharmacology, toxicology and pharmacokinetics studies in humans.