Because of differences in insulin binding of cultured lymphoid cell lines, T [thymus-derived] and B [bone marrow-derived] cell surface receptor and 125I-insulin binding studies were performed on the B and peripheral blood leukocytes of 13 children with active acute lymphocytic leukemia. Based on surface receptors, 9 patients had null cell disease, and 4 had T cell disease. The mean percent insulin binding of the B cells from the null cell patients was 10.0% .+-. 8.1, and 0.18% .+-. 0.13 from the T cell patients. The mean insulin binding of the cell suspensions of peripheral blood from the null cell patients was 7.3% .+-. 7.5 and 0.07% .+-. 0.06 from the T cell patients. Displacement studies with nonradioactive insulin indicated that null leukemic cells bore specific binding sites. These results indicated that there may be metabolic as well as surface membrane heterogeneity among acute lymphocytic leukemias of childhood.