A novel hematopoietic growth factor, the stem cell factor (SCF), for primitive hematopoietic progenitor cells has recently been purified and its gene has been cloned. In this study we tested the mitogenic activity of recombinant human SCF on myeloid leukemia cells as well as the expression of its receptor. We have investigated the proliferation of 31 myeloid leukemia cell lines as well as fresh myeloid leukemic blasts from 17 patients in a 72-hour 3H-thymidine uptake assay in the presence of various concentrations of recombinant human (rh) SCF alone or in combination with saturating concentrations of granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, M-CSF, interleukin-3 (IL-3), or erythropoietin (EPO). Only five of 31 lines, but fresh leukemic blasts from 12 of 17 patients with acute myeloid leukemia (AML), significantly responded to SCF. The responding cell lines were of the acute promyelocytic, chronic myeloid, megakaryoblastic, and erythroleukemia origin, the responding blast preparations of all French-American-British subtypes. Synergistic activities of SCF were found with G-CSF, GM-CSF, EPO, and IL-3. To determine the SCF binding sites on leukemic cells, we used 125I-radiolabeled SCF in Scatchard analysis and cross-linking studies. The leukemic cell lines responding to SCF expressed from 2,300 up to 29,000 binding sites per cell. The SCF receptor expression was downregulated in vitro by the presence of its ligand. Cross-linking studies demonstrated a 150-Kd SCF receptor on the surface of all responding myeloid leukemias. This study suggests that SCF may be an important factor for the growth of myeloid leukemia cells, either as a direct stimulus or as a synergistic factor for other cytokines. Furthermore, using polymerase chain reaction analysis of total RNA from the myeloid leukemia lines, we found expression of SCF-mRNA in 17 of 30 lines, suggesting autocrine mechanisms in the growth of a subgroup of leukemic cells by coexpression of SCF and its receptor.