A variety of surface markers, terminal deoxynucleotidyl transferase (TdT) activity, morphologic appearance, and cytochemical composition were studied in a group of 16 patients (13 children, 3 adults) with acute lymphoblastic leukemia (ALL). In 9 children, no surface markers were detected on lymphoblasts (null-type ALL). Leukemic blasts of 4 children formed E-rosettes. These E-rosette-forming blasts from 3 adult patients with ALL, were studied for the presence of Fc receptors. Of the leukemic blasts from these 7 patients, 2--76% expressed receptors for IgG Fc. Only 3 of 7 patients showed 9--42% receptors for IgM Fc. In addition, complement receptors were investigated in 6 of those 7 patients with T-cell ALL. Complement receptors were detected on 9--70% of the E rosette forming blasts from all 6 patients. TdT activity was elevated in T-cell ALL and in children with null-type ALL. The heterogeneity of Fc receptor expression on leukemic blasts in these patients demonstrates a malignant proliferation of T cells in different stages of differentiation or maturation. This observation might be helpful in subclassifying T-cell leukemias with regard to prognosis and the response to therapy.