The bioassay of erythropoietin in polycythemic mice was modified to include a protein-depletion diet and a divided erythropoietin injection schedule. Although less than 0.05 unit of standard erythropoietin was not detected, a more linear dose/response relationship resulted from increasing doses of erythropoietin in the 0.1 to 1.0 unit range. The amount of erythropoietin in concentrated specimens prepared from the 24-hour urinary excretion of 25 normal subjects was measured in comparison with known quantities of standard erythropoietin. A mean daily erythropoietin excretion in men of 2.8 ± 1.3 units, in women of 0.9 ± 0.4 unit and in prepubertal boys of 1.0 unit was calculated. The higher erythropoietin excretion in adult males may be secondary to a greater production of erythropoietin in this sex.