To determine the type and proportion of cases within that type of acute lymphoblastic leukemia (ALL) that has a natural killer (NK) cell phenotype, we examined leukemic blasts from 31 children with ALL (14 with T-ALL, 17 with non-T-ALL) for expression of antigens detected by NK-specific monoclonal antibodies Leu 11b, Leu 7, and 1G2 (an antibody we have developed that cross-reacts with Leu 7). None of the patients had leukemic blasts that reacted with Leu 11b. However, leukemic blasts from four T-ALL patients were 1G2+ and/or Leu 7+. Blasts from two of these had spontaneous lytic activity against standard NK target cell line K562; blasts from one killed K562 only when incubated with interferon; blasts from the other had no lytic activity against K562 but did manifest antibody-dependent cell-mediated cytotoxicity against antibody-coated cells from NK-resistant cell line SB. Blasts from all four Leu 7+ patients had L2 morphology. In one, the leukemic blasts had azurophilic cytoplasmic granules similar to those found in NK-enriched normal populations of large granular lymphocytes. These findings suggest that a significant proportion of T-cell acute lymphoblastic leukemias may be malignancies of NK cell origin.