Leukemic cells from 12 patients with lymphoblastic lymphoma (LBL) and T cell acute lymphoblastic leukemia (T-ALL) were studied to determine the inducibility of myeloid antigens in culture in the presence and absence of 12-O-tetradecanoylphorbol-13-acetate (TPA) in association with discrete phenotypic and genotypic analyses on these cells. The investigation revealed that leukemic cells corresponding to common or mature thymocytes were never induced to express any myeloid antigens, and showed rearrangements of T cell antigen receptor (TcR) beta and gamma chain genes. Concomitant examination on leukemic cells from mature T cell malignancies, including adult T cell leukemia (ATL), T cell chronic lymphocytic leukemia (T-CLL) and T cell non-Hodgkin's lymphoma (T-NHL), also failed to express myeloid antigens in culture. By contrast, one of the panmyeloid antigens, CD13 (MCS-2) antigen was induced on leukemic cells corresponding to early thymocytes in 5 out of 7 cases in TPA-added culture and in 3 cases even in TPA-free culture. All of these CD13 antigen inducible cases exhibited the germ line configurations of TcR beta and gamma chain genes except for one case of T-ALL with sole TcR gamma chain gene rearrangement. These findings suggest that primitive T cells, still not undergoing TcR gene rearrangements, retain the characteristics of multipotent progenitor cells to possess different lineage markers and are able to express myeloid antigen not exceptionally. Both phenotypically and genotypically immature thymocytes are considered to be less restricted in the differentiation pathway of hematopoietic cells committed to T cell lineage.