Cells from 20 patients with leukemia and 9 with solid tumors were assayed for the enzyme methylthioadenosine phosphorylase, which function in both purine and polyamine metabolism in rapidly dividing cells. As determined by autoradiography of viable cells, and by direct enzymatic analysis, samples from one individual with pre-T-cell acute lymphoblastic leukemia and one with common acute lymphoblastic leukemia were methylthioadenosine phosphorylase deficient. In contrast, other leukemias of similar antigenic phenotype, as well as normal peripheral blood lymphocytes, thymic lymphocytes, and normal bone marrow cells, had substantial methylthioadenosine phosphorylase activity. This evidence suggests that the complete absence of methylthioadenosine phosphorylase distinguishes some leukemic cells in vivo from their nonmalignant counterparts.