The c-kit proto-oncogene encodes a 145- to 160-Kd transmembrane tyrosine kinase, which is a member of the platelet-derived growth factor receptor family and is allelic with the murine white spotting locus (W). W mutations affect several aspects of hematopoiesis, most notably erythroid progenitors and mast cells. A monoclonal antibody, YB5.B8, had been raised against the leukemic blasts of a patient with M1-type acute myelocytic leukemia (AML) and it precipitates a 150-Kd cell surface glycoprotein from leukemic cells. The YB5.B8 epitope is expressed on mast cells, on up to 3% of normal mononuclear bone marrow cells, and it identifies a sub-group of AML patients with a poor prognosis. In view of similarities noted between the cell surface antigen identified by YB5.B8 and the c-kit protein product, we performed experiments to determine whether they are identical. c-kit RNA expression in the cell lines HEL (human erythroleukemia) and A172 (glioblastoma) was shown to parallel the expression of the YB5.B8 epitope in these lines as measured by flow cytometry. Immunoprecipitation analysis with anti-kit serum and YB5.B8 antibody indicated that the two antibodies identified proteins of identical size in HEL (155 Kd) and A172 (145 Kd) cells, and sequential immunoprecipitations with the kit and the YB5.B8 antibodies demonstrated that the two antibodies recognize the same molecule. The proteins identified by both the anti-kit and YB5.B8 antibodies displayed in vitro autophosphorylation activity in immune complex kinase assays. In addition, YB5.B8 was able to inhibit the binding of the kit ligand to HEL cells. These studies provide evidence that the YB5.B8 antigen and the c-kit protein product are identical and raise certain hypotheses regarding the role of c-kit in AML.