A modification of the protocol of cytarabine and thioguanine was used for remission induction in patients with acute nonlymphocytic leukemia. First, synchronization of the mitotically active leukemia cells was attempted by administering cytarabine alone in a dose of 5 mg/kg of body weight 24 hours prior to initiating daily therapy with both drugs. Second, in order to recover dormant leukemic cells into the proliferative pool, both drugs were continued without interruption and without regard to the suppression of the circulating leukocyte and platelet count until bone marrow aspirates, repeated daily if necessary, were free of leukemic blast cells. This usually resulted in severe but reversible bone marrow hypoplasia, and 16 of 21 patients (76%) so treated achieved complete hematologic remission.