A high-pressure-liquid chromatographic (HPLC) procedure for quantitative assay of cefotaxime (CT) and its major metabolite in serum of normal individuals, desacetyl cefotaxime (DACT), is described. It employs Lichrosorb RP-8, elution with phosphoric-acid-methanol and UV absorption at 310 nm. The method is optimized for cefotaxime and allows differentiation between the parent compound and the biotransformation product DACT. The lower assay sensitivity level of CT and DACT is 0.3 μ g/ml. Correlation between HPLC and microbiological assay with Escherichia coli or Proteus rettgeri of pooled serum with CT added is r = 0.99. The method is rapid; processing of one sample takes 17 min. Use of HPLC avoids the errors of microbiological assays which derive from the presence in patient sera of different ratios of CT and DACT. The apparent rate of serum elimination is linearly related to the sensitivity of the microbial assay indicator strain to DACT. There is synergistic antibacterial activity between CT and DACT regardless of relative minimum inhibitory concentrations of the agents.