Picibanil (OK432), a pharmaceutical preparation of a low virulent Su strain of Streptococcus pyogenes, possesses cell regulatory activity particularly in its ability to augment natural killer (NK) cell activity and to activate macrophages to exert a tumoricidal effect both in vitro and in vivo. It is effective in retarding and/or inhibiting the growth of three different tumors: MBL-2 lymphoma, M109 alveolar adenocarcinoma, and B16 melanoma. The antitumor effect is mediated through regulation of NK cells and macrophages, possibly by its ability to stimulate the production and secretion of interferon and interleukin 1 and 2. It is a very effective adjuvant for tumor cell vaccines that elicit cytotoxic T-cell responses. Following cytoreductive chemotherapy (Cytoxan) Picibanil treatment leads to an earlier reconstitution of both bone marrow cellularity and differentiation to granulocyte-macrophage colonies.