A rapid and reliable method for the quantitative determination of cefmenoxime in serum and urine by reversed phase high-performance liquid chromatography is described. Serum was deproteinized with acetonitrile. Urine was diluted with dilute acetic acid (17.5 mmol/l). Separations were performed in isocratic mode using a C18 type column and a precolumn packed with Perisorb RP/8. The eluant consisted of a mixture of acetonitrile and 25.0 mmol/l acetic acid in a ratio of 32/69 (vol/vol). In normal subjects cefmenoxime was well separated from endogenous compounds and various added drugs. Its complete separation was confirmed by selective degradation with beta-lactamase from Bacillus cereus and UV spectrophotometry. The detection limit was 0.3 mg/l at a detection wave-length of 254 nm. Peak areas gave linear results up to concentrations of 500 mg/l. Within-batch precision (coefficient of variation) ranged from 1.1 to 6.2%. Recovery rates varied from 99.0 to 103.3%. Results of a standard microbiological assay correlated well with those obtained by the present HPLC method. Eight healthy volunteers who were given a single intravenous dose of 1 g cefmenoxime excreted 86.3 +/- 5.8% of the unchanged drug within 24 h in urine.