Summary Increased serum erythropoietin levels were produced in mice, rats, rabbits, and a sheep in response to hypoxia or phenylhydrazine anemia. In all cases, including that of the rabbit, the serum erythropoietin was neutralized on addition of serum from rabbits previously immunized with an erythropoietin extract of human urinary origin. Also, an erythropoietin extract prepared from phenylhydrazine anemic sheep plasma was neutralized by such serum. These data suggest that erythropoietins of the various animal species studied are similar.