We previously reported the augmentation of tumor cytotoxicity of polymorphonuclear leukocytes (PMN) by in vivo administration of a streptococcal preparation, OK-432 (S. Watabe et al., J. Natl. Cancer Inst., 72: 1365-1370, 1984). The present study was undertaken to elucidate the mechanisms of the phenomena. Mouse and rat spleen cells were stimulated in vitro with OK-432. The culture supernatants...