A new immunomodulating agent, bestatin (INN: Ubenimex has low toxicity even after long-term oral administration and has significant modifications in immunological response. A cooperative randomized controlled study of bestatin immunotherapy in combination with remission maintenance chemotherapy for adult acute nonlymphocytic leukemia (ANLL) was performed. After induction of complete remission, patients were randomized to the bestatin group (30 mg/bw per os (po) daily) and the control group. The 101 eligible cases (bestatin: 48, control: 53) were analyzed; the bestatin group achieved longer remission than the control group and a statistically significant longer survival. Though this prolongation of remission was not significant in the bestatin group compared to the control group in the 15-49 yr age group, in the 50-65 yr age group it was significantly longer. Bestatin is shown to be a clinically useful drug for immunotherapy of adult ANLL, since it has prolonged survival and remission especially in elderly patients, with few side-effects.