l-Asparaginase was added to vincristine and prednisone for induction of first remission in 815 children with acute lymphocytic or acute undifferentiated leukemia. This combination resulted in an overall remission rate of 93%. The addition of l-asparaginase to the standard induction regimen using prednisone and vincristine did not significantly increase the morbidity or mortality rate during the induction period. The most common side effect was transient l-asparaginase-induced hyperglycemia. The safe administration of l-asparaginase i.m. and the dose efficacy of 6000 I.U./sq m were confirmed. For these reasons, l-asparaginase should be combined with vincristine and prednisone for the initial induction of children with acute lymphocytic or acute undifferentiated leukemia.