The receptor for erythropoietin (EpoR) is normally restricted in its expression to the relatively mature cells of the erythroid and megakaryocytic lineages. Using retrovirus-mediated gene transfer, the wild-type EpoR and a constitutively activated mutant of the EpoR, EpoRR129C, were expressed in primary hematopoietic cells. Retroviral infection of day-12 murine fetal liver, followed by stimulation with Epo as a single stimulus, generated day-8 erythroid colonies resembling colonies derived from burst-forming units-erythroid (BFU-E). Similarly, murine post-5 fluorouracil (5-FU) bone marrow cells or fetal liver cells, induced to express EpoR and stimulated by Epo, displayed a significant enhancement of megakaryocyte colony formation, particularly of the BFU-megakaryocyte (BFU-Mk) colony type. Cultures of bone marrow cells transduced with the EpoR retrovirus and stimulated by Epo contained macrophage colonies but very few granulocyte colonies. Experiments to culture single clones demonstrated direct action of Epo on megakaryocyte and macrophage clones but failed to demonstrate a direct action on granulocyte precursors. A similar pattern of lineage-restricted effects was demonstrated in unstimulated cultures of cells infected with the EpoRR129C retrovirus. In summary, we have demonstrated Epo-induced recruitment of immature erythroid and megakaryocyte precursors induced to express the EpoR. Furthermore, we have also demonstrated lineage-restricted cell proliferation in response to Epo by normal myeloid hematopoietic cells transduced with the EpoR.