Neocarzinostatin (NCS) is an antitumor antibiotic isolated from streptomyces carzinostaticus by Ishida et al. in Japan in 1957. The chemical structure of NCS is a single chain made of acidic polypeptide consisting of 112 amino acids. Its antitumor activities have been demonstrated in various experimental tumors. Its mode of action is considered to inhibit DNA synthesis at early stage of S phase and to block it at G(2) phase. The agent has been used clinically for solid tumors without remardable effects, although never been used in treating human leukemia. For the first time the author has given NCS alone to patients with acute leukemia in the dose ranging from 0.04-0.06 mg/kg/day for 4-5 days as one course of treatment. In case no sufficient remission was induced further courses of treatment were repeated after an internal of 4-10 days, and obtained the following results. 1) Of 18 cases 7 (38.9%) attained complete remission and 4 (22.2%) partial remission. 2) Complete remission was obtained even in the patients with refractory varieties of acute leukemia, i.e., acute promyelocytic leukemia, monocytic leukemia and leukemia in aged group. 3) No noteworthy side effects other than occasional nausea or anorexia were observed and suppression on normal hematopoiesis was very slight. From these results it can be concluded that NCS is one of the most potent antileukemic agents for inducing remission in acute leukemia.