The role of recombinant rat stem cell factor (rrSCF) was studied on defined primitive bone marrow cell populations. In agar culture of 500 lineage-negative/Sca-1-positive (Lin-/Sca-1+) cells, rrSCF alone stimulates small colonies of predominantly granulocytic cells. The combinations of rrSCF plus interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), or macrophage CSF (CSF-1) stimulated primitive progenitor cells defined as high proliferative potential colony-forming cells (HPP-CFC). Synergistic increases in total colony numbers were obtained with rrSCF plus GM-CSF, granulocyte CSF (G-CSF), CSF-1, or IL-6, but not IL-1 or IL-3. Lin-/Sca-1+ cells were incubated in liquid culture at 3,000 cells/mL for 6 days in the presence of rrSCF alone or in combination with other growth factors. The total number of cells was increased twofold in the presence of rrSCF, with the progeny primarily myeloid in nature. The greatest increase in cell number was obtained with rrSCF plus IL-3, where the cell number increased 40-fold. These factors also stimulated an increase in HPP-CFC (10-fold) and GM-CFC (500-fold). To determine if these interactions were direct, single Lin-/Sca-1+ cells were sorted into microtiter wells and the cell proliferation scored 6 days later. RrSCF synergized with IL-3, IL-6, and G-CSF to stimulate the proliferation of single cells. The cells in positive wells were subcultured into colony-forming assays and up to 400 CFC per well were obtained after 14 days incubation of the secondary cultures. These data demonstrate that rrSCF acts in combination with various growth factors to directly stimulate the amplification potential of hematopoietic primitive precursors, resulting in differentiation of these precursors.