Neocarzinostatin (NCS) is a polypeptide antibiotic which has been shown to have antileukemic activity in Japanese trials. Twenty-two patients, who were in relapse with acute nonlymphocytic leukemia refractory to other agents, were studied with two dose schedules of NCS. Schedule 1, 2000-8000 units/m2/day by continuous infusion for 5-10 days, was given to 12 patients and schedule 2, 2000-6000 units/m2/day by iv bolus every 8 hours, was given to ten patients. Clearing of peripheral blast cell was seen in nine of 12 patients treated with schedule 1 and in four of ten patients treated with schedule 2. Bone marrow effect was variable and appeared to be dose-related. No unequivocal complete remissions were seen with either schedule. NCS perturbations of cycling bone marrow cells were studied in two patients receiving the drug by continuous infusion. Both of these patients showed G2 accumulation of marrow cells following treatment. Immunochemically detectable levels of drug were seen in three patients treated with continuous infusion. In summary, NCS has antileukemic effect in patients with acute nonlymphocytic leukemia refractory to other agents. However, its role in the therapy for acute leukemia required further evaluation.