Induction of proliferation and differentiation in response to recombinant human interleukin-3 (hIL-3) was studied in liquid and semisolid cultures of umbilical cord blood and bone marrow cells that were fractionated by "panning" with anti-My10 antibody according to expression of CD34 antigen. Cells from enriched fractions (70% to 90% CD34+) were found to proliferate strongly in response to hIL-3. Phenotypic analysis and morphologic characterization of the proliferating cells demonstrated a rapid decrease in CD34+ cells and an exponential increase in the number of cells belonging to the neutrophilic, eosinophilic, monocyte/macrophage, and thrombocytic lineages. When combined with recombinant human erythropoietin, burst colonies and cells expressing glycophorin-A were detected, thereby demonstrating the effects of hIL-3 on erythroid progenitors. Further, the development of mixed-erythroid colonies indicated that multipotential cells within CD34-enriched fractions responded to hIL-3. In addition, we examined the effect of hIL-3 on the proliferation of primary acute myeloblastic leukemia cells in liquid culture. We found that hIL-3 was able to induce cell proliferation in a proportion of the cases tested. Heterogeneity of the responses to hIL-3 was in part related to French-American-British classification but could not be correlated with CD34 antigen expression by the leukemic cells. These results indicate that, although the effects of hIL-3 on proliferation and differentiation of cells obtained from normal hematopoietic specimens were primarily borne by CD34+ cells, expression of the CD34 molecule per se is an insufficient condition to determine a growth response to this lymphokine.