The c-kit proto-oncogene encodes a receptor tyrosine kinase that is crucial to hematopoiesis, melanogenesis, and gametogeneis. Although the enzymatic activity of the c-kit product (KIT) is regulated by its ligand, both the Val559-->Gly (G559) mutation in the juxtamembrane domain and the Asp814-->Val (V814) mutation in the phosphotransferase domain lead to constitutive activation of KIT. By retroviral infection of hematopoietic progenitor cells with KIT(G559) or KIT(V814), KIT(G559) induced development of granulocyte/macrophage and mast-cell colonies in vitro without the addition of exogenous growth factors. KIT(V814) induced factor-independent growth of various types of hematopoietic progenitor cells, resulting in the development of mixed erythroid/myeloid colonies in addition to granulocyte/macrophage and mast-cell colonies. Furthermore, transplantation of KIT(G559) and KIT(V814)-infected bone marrow cells led to development of acute leukemia in one of 10 and six of 10 transplanted mice, respectively. No mice developed hematologic malignancies after transplantation of wild-type KIT-infected cells. Furthermore, transgenic mice expressing KIT(V814) developed acute leukemia or malignant lymphoma. These results demonstrate a direct role of the mutant KITs, particularly KIT(V814), in tumorigenesis of hematopoietic cells and suggest that similar mutations may contribute to the development of human hematologic malignancies.