Using suspension cultures of purified bone marrow CD34+ cells, we have analyzed the effects of the combination of erythropoietin (Epo) and thrombopoietin (Tpo) on the in vitro differentiation toward erythropoiesis and thrombopoiesis. The number of CD41+ cells that accumulated over 2 weeks of culture, as well as the number of globin+ cells in the same cultures, was found to be significantly higher with the Epo+Tpo combination compared to either cytokine alone. No evidence was found that Tpo affected the differentiative action of Epo. Instead, there was a significant expansion of erythroid progenitors, both erythroid colony-forming and burst-forming units (CRU-E and BFU-E), by 7 days in culture, suggesting a proliferative effect of Tpo on erythroid cells in vitro. To determine the phenotypic features of erythroid progenitor cells which were targets of Tpo's action, and specifically to inquire whether the effect was directed mainly toward bipotent erythroid/megakaryocytic (E+Mk) progenitors, we isolated subsets enriched for both erythroid and megakaryocytic progenitors from CD34+ cells. We found that 1) BFU-E and CFU-Mk co-segregate in the subset of CF34+ cells that is negative for the phosphatase isoform CD45RA; 2) the presence of CD41 on this subset appears to segregate late erythroid and late CFU-Mk from early erythroid and early CFU-Mk, which are CD41-negative; 3) bipotent erythroid/Mk progenitors, studied by single-cell culture assays, were found mainly in the CD41+ and rarely in the CD41- subsets which included more multipotent progenitors; 4) by comparing the frequencies of pure erythroid or pure megakaryocytic progenitors to that of bipotent E+Mk progenitors, we conclude that the erythroid-enhancing effect of Tpo is directed mainly toward pure erythroid progenitors expressing CD41 and Mpl, as suggested by independent experiments employing anti-Mpl antibody, rather than only on bipotent E+Mk progenitors.