The degree of A → C hemoglobin switching following administration of a human urine erythropoietin preparation to a non-anemic sheep was compared, in the same animal, with the switching obtained with administration of anemic sheep plasma containing known amounts of erythropoietin. A strong correlation emerges between the dosage of erythropoietin given and the degree of A → C switching observed. This correlation between erythropoietin dose and A → C response is maintained despite the use of materials that originated in different species (man and sheep), were assayed for erythropoietin in yet another species (mouse), and had grossly different specific activities (units/mg.) of erythropoietin, so that the specific activity of the human urine preparation was approximately 400 to 1600 times that of anemic sheep plasma. We believe these observations provide substantial evidence for a provisional hypothesis of identity between erythropoietin and the A → C switching factor.