Twenty-eight patients with advanced acute nonlymphocytic leukemia, 16 with acute lymphoblastic leukemia, and two with acute undifferentiated leukemia were treated with ICRF-159. No patient achieved a complete remission and only three patients (6%) achieved a partial bone marrow remission. The only significant toxic effect was myelosuppression which probably contributed to the death of six patients. Five patients with acute nonlymphocytic leukemia and one with acute lymphoblastic leukemia received a combination of ICRF-159 and low-dose cytosine arabinoside. There were no remissions in this group and the toxic effects were more marked than with ICRF-159 alone. This study confirms the limited activity of ICRF-159 as a single agent in acute leukemia demonstrated in smaller series, and shows that, when used in combination with low-dose cytosine arabinoside, it was ineffective and resulted in increased toxicity. ICRF-159 alone or in combination with cytosine arabinoside has very limited activity in advanced adult acute leukemia.