The cytolytic function of natural killer (NK) cells and their responsiveness to interferon-α and IL-2 were investigated in children with acute lymphoblastic leukemia (ALL) using51Cr-release and single-cell assays. For comparison, such NK cell functions were similarly assayed in neuroblastoma. NK activity in ALL children was extremely low at onset, but it increased gradually during remission and finally reached normal levels. At the single-cell level, their NK cells at onset were defective in the binding, lytic, and recycling abilities. Although the binding and lytic defects improved to normal levels during remission, the recycling, which increased gradually during remission, was still low even after the long-term remission in ALL: the maximal recycling capacity values were 1.9 ± 0.4 (p< 0.001) at onset and 4.6 ± 0.6 (p< 0.05) after 5 y of complete remission, as compared to the value in control children of 5.4 ± 0.7. On the other hand, children with neuroblastoma had no recycling defect after completing the therapy: their maximal recycling capacity value was 5.6 ± 0.7. Bone marrow cells in ALL were also depressed in their recycling ability at all stages. Interferon-α and IL-2 could enhance NK activity and IL-2 could generate lymphokine- activated killer activity at all stages of ALL; however, the recycling defect hardly improved with these treatments. Thus, NK cells in childhood ALL have a recycling defect as a functional characteristic.