A standard method of concentrating urinary erythropoietin and a standard assay procedure can be used to demonstrate the hormone in the urine of normal human beings. The eythropoietically active material recovered produced increasing response to increasing dose and was completely neutralized by rabbit serum containing antibodies to human urinary erythropoietin. The average normal man excretes approximately 1 standard A unit of erythropoietin per day and the average normal woman excretes approximately 0.4 units per day. With the exception of one patient with a renal allograft, and normal subjects living at extremely high altitude, the recovery of erythropoietin from the urine has not been found to exceed normal in patients with polycythemia, whether the polycythemia is primary or secondary to renal pathology. Markedly elevated levels of erythropoietin were found in subjects living at extremely high altitude.