The simultaneous renal clearances of iothalamate sodium I 125, cyanocobalamin Co 57, and inulin were compared in 19 patients with a variety of renal diseases. The renal clearances of both iothalamate sodium 1125 and cyanocobalamin Co 57 are not statistically different from the clearance of inulin or from each other. Plasma protein binding was insignificant with iothalamate sodium I 125 but averaged 22% with cyanocobalamin Co 57 with a considerable variation between patients. These observations, as well as a lower cost per study, suggest that iothalamate sodium 1125 is an excellent radioactive material to substitute for inulin clearance when measuring the glomerular filtration rate in man.