Human granulocyte-macrophage colony-stimulating factor (GM-CSF) has been described as a multilineage growth factor that induces in vitro colony formation from erythroid burst-forming units (BFU-E), eosinophil colony-forming units (CFU-Eo), and multipotential CFU (CFU-GEMM) as well as from granulocyte-macrophage CFU (CFU-GM), granulocyte CFU (CFU-G), and macrophage CFU (CFU-M). In this paper we provide evidence indicating that GM-CSF, when tested for its stimulating capacities expressed upon highly enriched hematopoietic progenitor cells (CD34+/monocyte-depleted), is unable to induce colonies from CFU-GM, CFU-G, or CFU-M. Only BFU-E, CFU-Eo, and CFU-GEMM were stimulated, and thus GM-CSF induces a similarly restricted spectrum of progenitor cells as does recombinant human interleukin 3 (IL-3). We then compared the relative stimulating potencies of GM-CSF and IL-3 by measuring colony numbers of CFU-GEMM, BFU-E, and CFU-Eo generated from CD34+ progenitor cells. IL-3 and GM-CSF as single factors were equally active in stimulating CFU-GEMM, but the combination of both factors produced additive stimulative effects upon CFU-GEMM. IL-3 was a more potent stimulus of BFU-E, and GM-CSF was the more active stimulating factor for CFU-Eo. We conclude that GM-CSF and IL-3, although stimulating the outgrowth of identical types of progenitor cells, particularly differ as regards their comparative quantitative efficiency of stimulation.