Hematopoiesis is thought to be modulated by interactions of progenitor cells with hematopoietic growth factors. We have shown that colony- forming units-spleen (CFU-S) and repopulating stem cells require interleukin-3 (IL-3) to survive in vitro, and that CFU-S number and long-term repopulating ability can be increased by culture in the combination of IL-3 and IL-6. In this report, we describe the effects of stem cell factor (SCF) on CFU-S and repopulating stem cells. Injection of SCF into anemic Sl/Sld mice caused a twofold and 20-fold increase in CFU-S number in the bone marrow and spleen of treated animals, respectively. After 6 days in suspension culture, CFU-S number increased threefold in cultures supplemented with SCF and IL-6, or SCF, IL-3, and IL-6 relative to the number at day 0. The long-term repopulating ability of cells cultured in SCF, IL-3, and IL-6 was approximately sevenfold better than that of cells cultured in IL-3 or SCF. Similar experiments were performed on populations of bone marrow cells enriched for, or depleted of, CFU-S by elutriation and lineage subtraction. The combination of SCF and IL-6 increased CFU-S number approximately fourfold to eightfold in the CFU-S-enriched fraction, but had no effect on the CFU-S-depleted cells. These results show that SCF alone can increase CFU-S number in vivo, and in combination with other growth factors increases CFU-S numbers in vitro.