The streptococcal preparation, OK-432, has been found to be a biological response modifier. The effects of OK-432 on autologous CFUs in mice and hematopoietic disorders induced by anti-leukemic agents (DNR: daunorubicin, ara-C: cytosine arabinoside, NCS: neocarzinostatin) were investigated in order to obtain information as to the possibility of its clinical application in induction chemotherapy for acute leukemia. OK-432 increased autologous CFUs in irradiated mice, suggesting that it acted hematopoietically on multipotent stem cells. In mice treated with DNR, OK-432 reduced neutropenia when it was given before and simultaneously with DNR. In mice pretreated with OK-432, the duration of neutropenia caused by ara-C tended to be prolonged. On the other hand, simultaneous administration of OK-432 with ara-C showed no significant differences as compared to ara-C alone. These results suggested that the simultaneous administration of OK-432 with chemotherapeutic agents might protect normal hematopoiesis during induction chemotherapy for acute leukemia in man.