A high-performance liquid chromatographic assay for the analysis of josamycin in human serum and urine is presented. The assay involves a simple solid-phase extraction procedure coupled with a phase separation step, separation on a reversed-phase C18 column with UV detection by a multi-wavelength programmable detector. The mobile phase was acetonitrile-0.015 M phosphate buffer, pH 6.0 (5:2) at a flow-rate of 1.2 ml/min. The column temperature was maintained at 35 degrees C. Linear calibration curves over the concentration ranges 0.1-2.0 mg/l (serum) and 0.5-5 mg/l (urine) were obtained with correlation coefficients of 0.9983 and 1.0000, respectively. The relative standard deviations of five replicate samples at the upper and lower limits of each calibration curve were below 7%. The recoveries at the upper and lower ends of the calibration range for serum were 77% and 70%, respectively, and those for urine were 76% and 80%, respectively.