Thec-kilproto-oncogene encodes a receptor tyrosine kinase that is considered to play important roles in hematopoiesis. The proto-oncogenec-kitproduct is expressed on various types of human cell lines derived from leukemic cells of erythroid, megakaryocytic and mast-cell lineages. Also, thec-kitproduct is detectable in blast cells in most cases of acute myeloblastic leukemia (AML) and in some cases of chronic myelogenous leukemia (CML) in blastic crisis (BC). By contrast, little or no expression ofc-kitis observed in human leukemia cell lines of lymphoid lineage and in blast cells in acute lymphoblastic leukemia (ALL). Tyrosine phosphorylation and activation of thec-kitproduct with the ligand forc-kit(stem cell factor: SCF) results in proliferation of some human leukemia cell lines, such as M07E, and blast cells in a substantial fraction of AML cases. In addition, SCF appears to have an activity in inducing differentiation of certain types of leukemic cells. In some cases, further, thec-kitproduct is found to be activated in leukemic cells even before the stimulation with SCF. These results suggest thatc-kitmay be involved in excessive proliferation and aberrant differentiation of human leukemia cells.