Peripheral blood mononuclear cells (PBMC) from acute lymphoblastic leukemia (ALL) patients were studied for the expression of interleukin 2 receptors (IL-2R). When stimulated with phytohemagglutinin (PHA) for 3 days, the percentage of p55 Tac positive cells and the number of high affinity IL-2R was decreased in on-therapy ALL patients at remission and returned to normal after cessation of the treatment. However, the percentage of p75 IL-2R positive PBMC was normal in all ALL patients at remission. It is well-known that there are several immunological abnormalities including T and natural killer (NK) cell dysfunctions in ALL patients undergoing maintenance chemotherapy. IL-2 facilitates T-cell growth and corrects defective NK activity. Taking this into account, there may be a poor utilization of autologously produced IL-2 because of the impaired expression of IL-2R, and this poor utilization of IL-2 may be related to the defective immunological functions in childhood ALL patients at remission who are receiving maintenance chemotherapy.