The effects of recombinant human erythropoietin (rEp) on murine hematopoietic progenitors were studied using a serum-free culture. A high concentration of rEp stimulated the formation of mixed erythroid-megakaryocyte colonies (EM colonies) and blast cell colonies, as well as erythroid colonies, erythroid bursts, and megakaryocyte colonies from normal mouse bone marrow cells. Direct effects of rEp on EM colony, megakaryocyte colony, and erythroid burst formation were confirmed by depletion of accessory cells such as T cells, B cells, and macrophages from crude bone marrow cells, and inhibition of the colonies by the addition of rabbit anti-rEp antibody to the culture in a dose-dependent fashion. Replating experiments were performed to confirm the differentiating ability of blast cell colonies grown in the presence of rEp. Most of the blast cell colonies yielded not only secondary erythroid colonies but also megakaryocyte colonies in the presence of 2 IU/mL rEp. Some of the blast cell colonies produced secondary EM colonies in the presence of 16 IU/ml rEp of 2 IU/mL rEp plus interleukin-3, although no granulocyte-macrophage colonies were found in the secondary culture. These results suggest that Ep acts not only as a late-acting factor that is specific for erythroid progenitors, but also as a bipotential EM-stimulating factor for murine hematopoietic cells.