The effects of recombinant hemopoietic factors on the clonal growth of human megakaryocyte progenitors were explored using serum-free cultures of nonadherent and T-cell-depleted marrow cells. Recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) supported megakaryocyte colony formation in a dose-dependent manner, the activity being lower than that of recombinant interleukin 3 (rIL-3). Recombinant IL-3 and rGM-CSF acted synergistically on megakaryocyte colony formation when rGM-CSF was added to cultures containing suboptimal concentrations of rIL-3. However, the number and size of colonies did not increase with rGM-CSF when cultures were plated with an optimal dose of rIL-3. Recombinant erythropoietin (rEpo) by itself did not stimulate the growth of megakaryocyte progenitors. Recombinant Epo did, however, produce a significant increase in the number and size of megakaryocyte colonies in the presence of rIL-3 or rGM-CSF. Other factors, including recombinant granulocyte colony-stimulating factor, recombinant interleukin 1 alpha, recombinant interleukin 4, and recombinant interleukin 6 showed no capacity to generate or enhance megakaryocyte colony formation when added to cultures alone or in combination with varying concentrations of rIL-3. These results show that rIL-3, rGM-CSF, and rEpo affect human megakaryocytopoiesis by themselves or by interacting with each other.