In this study NCMP (neocarzinostatin, cytosine arabinoside, 6-mercaptopurine and prednisolone) two-step regimen with OK-432 was applied, to patients with ANLL in order to evaluate the clinical effects of the streptococcal preparation, OK-432. A total of 33 cases were randomized into two groups. Group A and group B were treated with NCMP two-step and NCMP two-step with OK-432, respectively. Group A consisted of 19 patients (16 AML and 3 AMoL) and group B consisted of 14 patients (13 AML and one AMMoL). All patients were previously untreated. The complete remission (CR) rate was 66.7% in group A and 64.3% in group B. However, patients in group B attained M(1) marrow and CR earlier than those in group A. Cytoreduction in peripheral blood and bone marrow was more rapidly obtained in group B than in group A. Normal hematopoiesis recovered more rapidly in group B than in group A. Cellular immunity (PPD) in group B tended to be maintained more than in group A. The CR rate in relapsed cases was higher in group B than in group A (44% vs 14%). Fever related to OK-432 was recognized in group B, but it was tolerable. OK-432 showed no prophylactic effects on infection. The simultaneous combination of OK-432 with chemotherapeutic agents might be expected to rapidly reduce leukemic cells, accelerate recovery of normal hematopoiesis and maintain cellular immunity during induction chemotherapy for ANLL.