Patients with acute leukemia in relapse were given 4''-(9-acridinylamino)methanesulfon-m-anisidide at a dosage of 75-200 mg/s m2 as a daily bolus infusion [subclavian central i.v.] on 5 consecutive days. None of the 11 patients treated at 75-150 mg/s m2 daily for 5 days achieved remission. Ten patients with acute lymphoblastic leukemia and 21 with acute nonlymphoblastic leukemia were given treatment at 200 mg/s m2 daily for 5 days. Six of these patients achieved complete remission, 3 with acute lymphoblastic leukemia and 3 with acute nonlymphoblastic leukemia. Neutropenia and thrombocytopenia were seen in all patients and in the responders lasted a median of 39 and 41 days, respectively. Stomatitis was the most significant nonhematopoietic toxicity noted, occurring in 80% of the patients. Hyperbilirubinemia was seen in 25% of the patients treated. Since 4''-(9-acridinylamino)methanesulfon-m-anisidide will induce remission in heavily pretreated patients with acute leukemia, consideration should be given to exploring its use in combination with other active drugs.