Type I diabetes mellitus (TIDM) is an autoimmune disorder characterized by T cell-mediated destruction of insulin-producing β cells in the pancreas. In the nonobese diabetic (NOD) model of TIDM, insulitis and diabetes are dependent on the presence of B lymphocytes; however, the requirement for specificity within the B cell repertoire is not known. To determine the role of Ag-specific B cells in TIDM, V H genes with different potential for insulin binding were introduced into NOD as H chain transgenes. VH125 H chain combines with endogenous L chains to produce a repertoire in which 1–3% of mature B cells are insulin specific, and these mice develop accelerated diabetes. In contrast, NOD mice harboring a similar transgene, VH281, with limited insulin binding develop insulitis but are protected from TIDM. The data indicate that Ag-specific components in the B cell repertoire may alter the course of TIDM.