OK-432 (a streptococcal preparation) has been widely used for cancer immunotherapy in Japan. It is the most potent immunomodulator in activating both macrophages and killer T cells and in increasing interleukin 2 production. Two K.E. (Klinische Einheit, clinical unit) of OK-432 were given intraperitoneally to each of 17 female nonobese diabetic (NOD) mice every week from 4-24 wk of age. NOD mice as well as BB rats spontaneously develop type I diabetes. During administration of OK-432, the development of diabetes was inhibited in 17 of 17 mice over the 24-wk observation period, whereas 14 of 17 female NOD mice given physiological saline had developed diabetes by 24 wk of age. At the onset of diabetes, nonfasting blood glucose was 511 +/- 82 mg/dl. Histologic examination showed that in the OK-432-treated NOD mice, 98% of total islets were intact or mildly infiltrated with mononuclear cells, whereas in saline-treated NOD mice, 79% of total islets exhibited severe insulitis. In OK-432-treated NOD mice, both the number of the mononuclear spleen cells and their natural killer cell activity was significantly increased.