Inhibitors of sodium transport purified from serum and urine of healthy and uremic subjects show the same elution pattern on columns of a weak anion exchanger (DEAE Sephadex A-25), reverse-phase chromatography (Separon SI C 18) and gel permeation chromatography (Separon Hema 300 Glc). Moreover, their thin-layer chromatographic mobility (DC-Alufolien Cellulose) in nine solvent systems is the same. These identical physicochemical properties document satisfactorily their identity, which permits the use of the urinary inhibitor of sodium transport for structure studies.