Mice with W mutations characterized by hypopigmentation, sterility, anemia, and mast cell deficiency have abnormalities in c-kit, a receptor with tyrosine kinase activity. Recently, the ligand for c-kit was cloned by investigators in several laboratories. Zsebo et al identified and cloned a gene for a cytokine termed stem cell factor (SCF) in the medium conditioned by buffalo rat liver cells, and this cytokine proved to be c-kit ligand. We have examined the effects of recombinant rat SCF (rrSCF) on colony formation from primitive hematopoietic progenitors in culture. rrSCF and erythropoietin (Ep) supported formation of granulocyte/macrophage (GM) colonies as well as a small number of multilineage and blast cell colonies from marrow cells of normal mice. We then examined the effects of rrSCF using marrow and spleen cells of mice that had been treated with 150 mg/kg 5- fluorouracil (5-FU). Unlike single factors, combinations of factors such as rrSCF plus interleukin-3 (IL-3), rrSCF plus IL-6, and rrSCF plus granulocyte colony-stimulating factor (G-CSF) markedly stimulated the growth of multilineage colonies. In contrast to these factor combinations and a combination of IL-3 and IL-6, a combination of rrSCF and IL-4 did not support multilineage colony formation. Mapping studies of the development of multipotential blast cell colonies further indicated that rrSCF, like IL-6, G-CSF, and IL-11, shortens the dormant period in which the stem cells reside. When we tested the effects of rrSCF using pooled blast cells, which are highly enriched for progenitors and are devoid of stromal cells, rrSCF plus Ep supported formation of only a few multilineage colonies, indicating that rrSCF itself is ineffective in support of the proliferation of multipotential progenitors. However, rrSCF supported formation of a significant number of neutrophil and neutrophil/macrophage colonies from pooled blast cells, indicating that rrSCF is able to support directly the proliferation of progenitors in neutrophil/monocyte lineages. c-kit ligand may play important roles in adult hematopoiesis.