The immunoregulatory activity of bone marrow leukemic blasts from five patients with null-cell acute lymphoblastic leukemia (ALL) and two children with T-cell ALL was evaluated. Blasts were studied in co-culture for their effects on the proliferative responses of normal allogeneic peripheral blood lymphocytes to phytohemagglutinin. Mitomycin C-treated null cell ALL blasts from two of five children suppressed the proliferative responses of normal responder cells by 80% and 58% whereas those from two other children enhanced the responses by 118% and 31%. T-cell leukemic blasts from one patient with T-cell ALL exhibited helper cell activity (26%) and T-leukemic blasts from the other demonstrated suppressor cell activity (53%). The helper cell activity of leukemic blasts was associated with stimulating capacity of null blasts in one-way mixed lymphocyte reactions. In six of seven cases, incubation of blast cells with concanavalin A (20 microgram/ml) for 48 hours prior to co-culture with responder cells did not result in the generation of significant suppressor cell activity. Our study suggests that leukemic blasts from certain patients with 'null' and T-cell ALL may possess spontaneous helper or suppressor cell immunoregulatory activity. This functional heterogeneity of leukemic blasts may help in subclassifying the ALL.