The effects of OK-432 (streptococcal preparation) on murine fulminant hepatitis were investigated. Hepatitis was induced by injection of mouse hepatitis virus type 2 (MHV-2) at a strength of either 1 x 10(3) or 1 x 10(4) plaque-forming units (PFU). Mice without OK-432 treatment died within 5 days, whereas mice preinoculated with OK-432 showed survival rates of 50% (1 x 10(3) PFU) or 10% (1 x 10(4) PFU) after 60 days. Survival time was not prolonged if OK-432 was injected after MHV-2. Examined histologically, mice not treated with OK-432 showed severe haemorrhagic necrosis of the liver, often panlobular. Treated mice showed less necrosis; the least necrosis was observed in those injected with OK-432 before MHV-3. In those mice injected first with OK-432 and then with 1 x 10(3) PFU of MHV-2 that survived 7 days, autopsy showed a very slight and focal hepatic necrosis, with follicular infiltration by lymphocytes and macrophages. Mitogenic reaction of spleen cells was remarkably less than normal in mice with MHV-2 injection. However, mice injected with OK-432 before MHV-2 (same treatment as mice showing high survival rates) showed relatively high reactivity in comparison with mice not treated with OK-432.