The pharmacokinetics of cefoxitin were examined in 4 healthy volunteers, 6 patients with normal renal function (inulin clearance, greater than 80 ml/min per 1.73 m2), and 35 patients with various degrees of renal insufficiency (inulin clearance, 80 to less than 5 ml/min per 1.73 m2). A single dose of 30 mg of cefoxitin per kg of body weight was injected intravenously over 3 min. Antibiotic concentrations in plasma were determined by the agar diffusion technique. The cefoxitin half-life increased progressively from 39 min in subjects with normal renal function to 23.5 h in oligoanuric patients. Correspondingly, total body clearance decreased from 340 to 13 ml/min per 1.73 m2. In addition to the study of cefoxitin kinetics, in 29 of the 41 patients, three different renal clearance tests were performed (inulin, p-aminohippurate, and creatinine clearances). Of these, p-aminohippurate clearance showed the best correlation with the elimination rate constant beta as well as total body clearance of cefoxitin; but with respect to beta, the differences between the p-aminohippurate and creatinine clearances were quantitatively negligible. Therefore, even in substances which are eliminated predominantly by active tubular secretion, creatinine clearance could be recommended for dosage adjustments.