The purpose of this study was to define the effects of recombinant human Steel factor (rhSF) (c-kit ligand) on early and late cord blood hematopoietic progenitors. In the presence of recombinant human erythropoietin (rhEpo), rhSF supported the development of granulocyte/macrophage, erythroid, and mixed colonies from CD34-positive cord blood cells. With increasing concentrations of rhSF, an increase in the number of mixed colonies was observed. This increase was paralleled by a decrease in the number of erythroid colonies, such that the total number of the two colony types was always constant. Similar results were observed when CD34+ cells were cultured in the presence of a combination of rhEpo, rhSF, and rhIL-3. These results indicate that the presence of rhSF enhanced the detectability of the nonerythroid components of mixed colonies. The effect of rhSF on early progenitors (pre-colony-forming units [pre-CFU]) was studied in a two-step assay. In this assay, cells positive for CD34 and resistant to treatment with 4-hydroperoxycyclophosphamide (CD34+/4-HCres) were cultured in suspension for 7 days with rhSF, rhIL-3, or rhSF plus rhIL-3 and then plated in clonogenic assays. Before suspension culture, CD34+/4-HCres cells had a low clonogenic potential (0.37%). After being cultured in suspension, however, these cells gave rise to a large number of colonies of all types when replated. Cells cultured in suspension with the combination of rhSF and rhIL-3 increased significantly in number and gave rise to more colonies, when compared to cells cultured with either factor alone (synergistic effect). In identical experiments, such as effect had not been observed with combinations of rhIL-3 and either rhIL-1 or rhIL-6. Thus, rhSF supports the expansion and differentiation of early progenitors in human cord blood.