Numerous attempts have been made to increase the efficacy of penicillin in the treatment of infections by altering the rate of its absorption or excretion. Beyer and his associates have reported that caronamide (4′-carboxyphenylmethanesulfonanilide), though excreted only by the glomeruli, alters the specific renal tubular enzyme system responsible for the excretion of penicillin.1The excretion of penicillin is delayed, so that a higher concentration may be maintained in the blood for more prolonged periods. Other investigators have confirmed the original reports on the penicillin-enhancing effect of caronamide and have noted that the drug is relatively nontoxic when administered over short periods of time.2 There is difference of opinion concerning the dose of caronamide necessary to maintain an effective concentration of penicillin in the blood. Direct information on the absorption and excretion of caronamide in human beings has been lacking, since practical methods of measuring the concentrations of the