The clinical effectiveness of a new protocol, the NCMP (N:neocarzinostatin C:cytosine arabinoside M:6- mercaptopurine P:prednisolone) two-step regimen was studied in 19 patients with acute non-lymphocytic leukemia (ANLL). All patients were previously untreated and ages ranged from 15 to 75 y.o. (median:54 y.o.). The ratio of male to female was 11 to 8. The doses and administration-routes of antileukemic agents were: N, 1,400u/m(2)/day, was administrated by i.v. drip on day 1, 2, 3 and 4 (Regimen 1) and N, 4,000u/m(2)/day, i.v. drip on day 1 and 2 (Regimen 2). C, 60-100mg/m(2)/day, was administered by i.v. drip and M, 60-100mg/m(2)/day, was given p.o. daily. P, 20-40mg/m(2)/day, was also given p.o. daily. As the first step, antileukemic agents were administrated until peripheral blood cell counts and bone marrow nucleated cell counts decreased to less than 1,200/cmm and 15,000/cmm, respectively. Treatment with or without N as the second step was started about 3-7 days after the first step treatment. Eighteen patients were adequately treated by the NCMP two-step regimen and 12 of 18 patients, 66.7%, obtained complete remission (CR). On the basis of the types of leukemia, CR was achieved in 11 of 16 patients with AML, 68.8%, and 1 of 2 patients with AMoL, 50%. The ratio of CR was higher in younger patients (less than 49 y.o.) than in elderly patients (more than 50 y.o.). These ratios were 85.7% and 54.5%, respectively. On the basis of therapeutic regimens, 8 of 11 patients, 72.7%, achieved CR by Regimen 1 and 4 of 7 patients, 57.1%, by Regimen 2. The duration of CR ranged from 1.0 to 39.5+M (median:11.5 M). The survivals from the diagnosis of leukemia were from 13 to 41+M (median:12.1 M) in all patients and from 4.2 to 41+M (median:16.4 M) in responders. As toxic manifestations, digestive side effects such as anorexia, 68.4%, and nausea and vomiting, 21.1%, were marked, but were tolerable. These results indicate that the NCMP two-step regimen is an effective induction chemotherapy for ANLL.