Leukemic cells from the peripheral blood of 52 patients with acute and chronic leukemias were incubated with 12-0-tetradecanoyl phorbol ester (TPA). Thirty-one cases of lymphocytic leukemia (18 cases of acute lymphoblastic and 13 cases of chronic lymphocytic leukemia), 13 cases of acute nonlymphoblastic (myelo or myelomonoblastic) leukemia, and eight cases of blastic crisis of CGL (seven cases of predominantly myeloblastic crisis, and one case of lymphoblastic crisis) were studied. In all cases of lymphoid leukemia, cells formed clumps or aggregates after exposure to TPA, while in all cases of myeloid leukemia cells became adherent to the substrate. Seven of the eight cases of blastic crisis of CGL were predominantly myeloid in type and cells adhered to the substrate, while in a single case of lymphoid crisis in CGL cells formed clumps after TPA exposure. Functional, cytochemical, and ultrastructural studies showed altered cell differentiation and continuing in vitro maturation of leukemic cells after exposure to TPA. In the light of the above results, it is concluded that this simple test employing TPA exposure in vitro serves as a reliable means of distinguishing blasts from different origins in human leukemias.