A simple and sensitive high-performance liquid chromatographic micro-method for the determination of roxithromycin in human plasma and urine is described. A dichloromethane extract of the sample was chromatographed on a C18 reversed-phase column with acetonitrile-83 mM ammonium acetate-methanol (55:23:22, v/v) adjusted to pH 7.5 with acetic acid as the mobile phase. Roxithromycin and the internal standard, erythromycin, were detected by dual coulometric electrodes operated in the oxidative screen mode. The applied cell potential of the screen electrode was set at +0.7 V and the sample electrode at +0.9 V. The intra- and inter-assay coefficients of variation were less than or equal to 7.0%. The detection limit (signal-to-noise ratio = 3) was 0.1 microgram/ml for both plasma and urine. A study of drug stability during sample storage at 4, 20 and 37 degrees C showed no degradation of roxithromycin. The method is convenient for clinical monitoring and pharmacokinetic studies.