In a pharmacolinetic study on a new semisynthetic penicillin, alpha-sulfobenzylpenicillin, sulbenicillin, serum level, serum half-life, apparent distribution volume, renal clearance, urinary excretion, and metabolism were determined after a 4-gm intravenous dose and compared to that of carbenicillin in 5 patients with normal renal function. In the case of sulbenicillin, the mean serum concentration at 1 hr was 157 plus or minus 25 mug/ml, the mean serum half-life was 70 plus or minus 10 min, the renal clearance was 95 plus or minus 25 ml/min, and the total urinary recovery after 24 hr was about 80% of the dose. The only metabolite detected in the urine was the penicilloic acid derivative, in an amount usually less than 5% of the dose. Serum values, serum half-live, renal clearances, and excretion pattern did not differ significantly from that of carbenicillin. In 8 patients with decreased renal function (creatinine clearance less than 50 ml/min) there was an inverse correlation between creatinine clearance and serum half-life.