The preventive effect of an immunopotentiator, beta-1,6;1,3 D-glucan, on the development of diabetes and insulitis was studied in BB rats. The intravenous administration of 1 mg kg-1 week-1 of beta-1,6;1,3 D-glucan from the age of 4 weeks decreased the cumulative incidence of diabetes from 43.3% (13/30) to 6.7% (2/30) (P less than 0.005) and also the incidence of insulitis from 82.4% (14/17) to 26.3% (5/19) at the age of 20 weeks (P less than 0.002). Eight of nine rats were free from diabetes for 5 weeks after stopping beta-1,6;1,3 D-glucan at the age of 20 weeks. The total numbers of leukocytes in the peripheral blood and spleen of the rats were increased by beta-1,6;1,3 D-glucan treatment (P less than 0.05), whereas their T lymphocytes subsets were not changed. These data indicate that immunopotentiators could modulate the autoimmune mechanisms directed to pancreatic islets and inhibit the development of diabetes in BB rats.