The ligand for c-kit, known as stem cell factor, mast cell growth factor, or kit ligand, plays a central role in normal hematopoietic stem cell, melanocyte, and gametocyte development and function during embryogenesis and in adult life. In vitro, stem cell factor promotes the survival of hematopoietic progenitors and enhances their proliferation in response to specific growth factors. Administration of recombinant soluble stem cell factor to rodents, dogs, and baboons produces a broad array of effects on hematopoiesis, though not all lineages are equally stimulated. At doses of more than 100 micrograms/kg/d stem cell factor stimulates neutrophilia, lymphocytosis, basophilia, and reticulocytosis and increases mast cells in multiple tissues. In vivo mast cell activation can occur. Marrow cellularity is increased and progenitor cells are increased in marrow, spleen, and blood, and marrow-repopulating cells are increased in the circulation of stem cell factor-treated animals. Stem cell factor synergizes with other hematopoietic growth factors in vivo. Low-dose stem cell factor, 25 micrograms/kg/d, that does not elicit a detectable biological response, enhances the effects of granulocyte colony-stimulating factor in vivo, increasing the neutrophilia and circulation of progenitor and marrow-repopulating cells above that which is achieved with either factor alone. In phase I human trials, dose-limiting toxicities, related to mast cell activation, were reached at 25 to 50 micrograms/kg/d of recombinant human stem cell factor. At these doses, progenitor and long-term culture-initiating cells are increased in marrow and increases in circulating levels of progenitor cells of multiple types are observed. Phase I-II trials of low-dose stem cell factor in combination with granulocyte colony-stimulating factor show that the combination increases the circulation of CD34+ cells and colony-forming progenitor cells. Further studies are needed to determine the therapeutic role of stem cell factor and its effects on expansion and maintenance of hematopoietic stem cells in vivo.