Environmental agents have been implicated in the pathogenesis of insulin dependent diabetes (IDD). These studies were designed to learn if dietary protein influences the development of IDD in the BB rat. Specifically, analysis involved the effects of substituting a modified, semi-synthetic diet (AIN-76) containing soy protein as the sole protein source for the standard chow containing a mixture of animal and non-animal protein. IDD was less frequent (73% vs. 38%, P less than or equal to 0.01), and the onset of diabetes was retarded (110 +/- 11.0 vs. 92 +/- 15.5 days, P less than or equal to 0.01) in rats fed the study diet versus standard chow, respectively. The frequency of thyroid collodal autoantibodies was also significantly decreased in rats fed the study diet (56% vs. 23%, P less than or equal to 0.04), whereas frequencies of smooth muscle and gastric parietal cell autoantibodies were less frequent, but not significantly so. Lymphocyte counts and subsets were unaffected. In non-diabetic rats at greater than 180 days of age, insulitis was less severe in the experimental group. These findings suggested that dietary protein may influence the development of IDD in the BB rat.