Normal lymphocytes lack insulin receptors. The binding of this hormone was studied in twelve patients with acute lymphoblastic leukaemia (ALL) and fourteen patients with chronic lymphocytic leukaemia (CLL). Lymphoblasts from ALL have been found to possess hormone receptors with properties identical to those of the known target cells for insulin: specificity, pH and temperature dependence, and ligand-induced increase in dissociation rate. All patients with ALL displayed insulin receptors on their lymphoblasts. The null-type cells possessed higher numbers of binding sites than the T-type cells, without overlapping of the values. In the fourteen patients with CLL, eight had low levels of insulin receptors on their lymphocytes while six showed a complete lack of such binding sites. Our results suggest that measurement of insulin binding might be a useful non-immunological marker for the classification of human leukaemias.