Because of differences in insulin binding of cultured lymphoic cell lines, T- and B-cell surface receptor and 125I-insulin binding studies were performed on the bone marrow and peripheral blood leukocytes of 13 children with active acute lymphocytec leukemia. Based on surface receptors, nine patients had null-cell disease and four had T-cell disease. The mean per cent insulin binding of the bone marrow cells from the null-cell patients was 10.0% +/- 8.1 and from the T-cell patients was 0.18% +/- 0.13. The mean insulin binding of the cell suspensions of the 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 the acute lymphocytic leukemias of childhood.