In vivo and in vitro bioassays, radio immunoassays (RIA), enzyme linked immunoassays (ELISA) and immunoradiometric assays (IRMA) have been applied to measure serum erythropoietin (Epo) levels. Results are expressed in international units (IU). In vivo bioassays are time consuming, expensive and not sensitive enough to measure the amount of Epo in normal sera. Nevertheless, this test remains the standard of comparison for the other new assays. Immunologic technics are specific, sensitive, reproducible, easy to perform, rapid and are now preferred for the measurement of Epo. Published values for non anemic human range from 6 to 32 IU/l. In the search of etiology of a polycythemia, the results of usual investigations to class the polycythemia either as primitive (P. vera) or secondary are often inconclusive. In these situations, measurement of serum Epo could be of value for accurate a differential diagnosis. However the results of the literature and our personal experience, show that the serum Epo level does not discriminate with an absolute fiability between Polycythemia Vera and secondary polycythemia because of a great overlap between the two groups.