Insulin-dependent diabetes is heavily influenced by genes encoded within the major histocompatibility complex (MHC), positively by some class II alleles and negatively by others. We have explored the mechanism of MHC class II–mediated protection from diabetes using a mouse model carrying the rearranged T cell receptor (TCR) transgenes from a diabetogenic T cell clone derived from a nonobese diabetic mouse. BDC2.5 TCR transgenics with C57Bl/6 background genes and two doses of the H-2^g7 allele exhibited strong insulitis at ∼3 wk of age and most developed diabetes a few weeks later. When one of the H-2^g7 alleles was replaced by H-2^b, insulitis was still severe and only slightly delayed, but diabetes was markedly inhibited in both its penetrance and time of onset. The protective effect was mediated by the Aβ^b gene, and did not merely reflect haplozygosity of the Aβ^g7 gene. The only differences we observed in the T cell compartments of g⁷/g⁷ and g⁷/b mice were a decrease in CD4⁺ cells displaying the transgene-encoded TCR and an increase in cells expressing endogenously encoded TCR α-chains. When the synthesis of endogenously encoded α-chains was prevented, the g⁷/b animals were no longer protected from diabetes. g⁷/b mice did not have a general defect in the production of A^g7-restricted T cells, and antigen-presenting cells from g⁷/b animals were as effective as those from g⁷/g⁷ mice in stimulating A^g7-restricted T cell hybridomas. These results argue against mechanisms of protection involving clonal deletion or anergization of diabetogenic T cells, or one depending on capture of potentially pathogenic A^g7-restricted epitopes by A^b molecules. Rather, they support a mechanism based on MHC class II–mediated positive selection of T cells expressing additional specificities.