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).