In vitro cultures of erythroid progenitors and radioimmunoassay of erythropoietin (Epo) are 2 recently available techniques. It is possible to assess their relevance in various hematological disorders. Erythroid cultures can be performed in the investigation of polycythemias, pure red cell aplasias (PRCA) and refractory anemias. In primary polycythemias "spontaneous" colonies appear in vitro whereas this phenomenon is never observed in secondary polycythemias. These so called "spontaneous" colonies have been demonstrated with a lower incidence in all myeloproliferative disorders. Therefore, if the absence of spontaneous colonies does not permit us to eliminate the presence of a myeloproliferative syndrome aside from polycythemia vera, their presence does seem pathognomonic of a myeloproliferative disorder. In acquired chronic pure red cell aplasia in adults, a strong correlation is found between the in vitro growth of erythroid colonies and the results of immuno-suppressive treatment. In refractory anemias erythroid cultures do not have either diagnostic, or prognostic interest. Serum epo level does not have a high discriminatory value in distinguishing between primary and secondary erythrocytosis. Indeed in PV, the Epo level is generally low or normal, in secondary polycythemias Epo level is high or normal. There is an important overlap between the two groups. Epo level determination can have a therapeutic incidence. Administration of recombinant Epo seems justified only in patients both sufficiently anemic to warrant transfusions and in whom Epo level is low in comparison with the degree of anemia.