Knowledge of the endogenous blood level of erythropoietin (Epo) has gained recent interest in view of the advances in Epo replacement therapy in anemic patients. By radioimmunoassay, we have carried out comparative measurements of the serum Epo level in patients suffering from chronic enterocolitis or leukemia. In chronic enterocolitis, the Epo level showed an exponential increase with the degree of anemia (up to 250 U Epo/1 serum at 70 g hemoglobin/1 blood). Similarly anemic patients with leukemia and severe bone marrow insufficiency of erythropoiesis had much higher Epo levels (usually above 500 U/1). Our findings indicate that the level of Epo is not only dependent on the blood hemoglobin concentration but also on the type of anemia. In fact, additional in vitro studies showed that immunomodulatory peptides can significantly influence the production of Epo in the hepatoma cell culture HepG2.