The results are reported of a study on the E.S.F. (erythropoietic‐stimulating‐factor) in serum from 47 patients with leucosis and 11 patients with other haematological disorders, and a survey is given of previous studies of the same natureThe analyses were performed over a period of 2 yearsFe59incorporation in the erythrocytes in starved rats was used as a criterion of the E.S.F. content in serumIn patients with chronic lymphatic leucosis and acute leucosis, a reciprocal relationship was found between the E.S.F. titre and the haemoglobin percentage, and a similar reciprocal relationship was found between the E.S.F. titre and the bone marrow erythropoiesis. This must be interpreted to mean that there are other factors which determine these patients' defective erythropoiesisIn patients with chronic myeloid leucosis, low values of E.S.F. were found in serum in spite of often reduced haemoglobin levels. Patients with high titres were observed, however, but in these cases the extramedullary erythropoiesis was inhibited, partly because of splenectomy, partly caused by roentgen treatment of the spleen. It seems reasonable to assume that the extramedullary, often ineffective splenic erythropoiesis requires E.S.F., and this might result in the low serum level and perhaps indirectly deprive the bone marrow of its natural stimula