We previously reported that streptococcal preparation (OK-432), which is a TNF inducer, inhibits insulitis and development of autoimmune diabetes in nonobese diabetic (NOD) mice and Bio-Breeding (BB) rats, as animal models of insulin-dependent diabetes mellitus. We have recently shown that recombinant human (h)TNF-alpha also suppresses development of diabetes in NOD mice. In this study we have extended our observation on TNF to BB rats in order to see whether TNF generally inhibits autoimmune diabetes. A total of 5 x 10(4) U of rhTNF-alpha was administered i.p., twice a week to male and female BB rats from 4 to 27 wk of age. The cumulative incidence of diabetes by 27 wk of age in nontreated rats was 36.4% (8/22), whereas that in hTNF-alpha-treated rats was 0% (0/21) (p less than 0.001). The hTNF-alpha-treated rats did not lose body weight and maintained normal blood glucose concentrations. Immunologic and histologic examinations were performed at the end of the experiment. Spleen cell cytotoxicities for NK-sensitive YAC-1 and rat insulinoma (RINm5F) cells in hTNF-alpha-treated rats significantly decreased in comparison with nontreated and nondiabetic BB rats. Intensity of insulitis was also inhibited in hTNF-alpha-treated rats. Interestingly, a huge hepatomegaly and splenomegaly was found in two of the 21 hTNF-alpha-treated rats. The latter consisted of W3/13dull+ and W3/25dull+ cells, which did not exhibit cytotoxicity for either YAC-1 or RINm5F cells. These results indicate that the chronic and systemic administration of TNF has a regulatory role in autoimmune diabetes in BB rats as well as in NOD mice, and that these animals may have a defect in TNF-mediated immunoregulation.