A rapid, specific and precise method is described for the analysis of cefoxitin in plasma and urine by reversed-phase, high performance liquid chromatography (HPLC). Cefoxitin and the internal standard, 3-isobutyl-1-methyl xanthine are eluted after 5.3 and 7.5 min, respectively. The assay sensitivity limit is 1 to 2 mg/l of cefoxitin sodium at 254 nm. Commonly prescribed antibiotics do not interfere. The assay is suitable for routine monitoring and pharmacokinetic studies of cefoxitin.