A male sex-dependent protein fraction (MUP), consisting of four closely related proteins, was isolated from the urine of male Wistar rats. The amino acid composition of two of the components is described. Quantitation of this protein in plasma was performed by the Mancini technique. It was found that after nephrectomy the plasma level of this protein increased rapidly. The metabolic fate of this protein was studied with the aid of 131I labelled MUP. It was found that the kidneys actively concentrate this material, and excrete it into the urine. Among the organs, the skin seems to contribute particularly to the metabolic fate of MUP.