A rapid and accurate high speed liquid chromatographic method has been developed for the determination of cephalexin in human plasma and urine. The method involves micropore filtration of urine specimens and methanol extraction of plasma samples followed by HSLC separation on a bonded reverse phase column utilizing a mobile phase of methanol-water containing acetic acid. The quantitativity of UV response at 254 nm covered a wide range of cephalexin concentrations down to 0.5 microgram/ml, and no metabolite peaks were detected. The time courses of plasma level and urinary excretion were determined until 6 hours after oral administration of cephalexin capsules to healthy volunteers. The pharmacokinetic parameters were estimated using a two compartment open model.