Kilham rat virus (KRV) infection of BB/Wor diabetes-resistant (DR) RT1u rats induces autoimmune diabetes without direct cytolytic infection of pancreatic β-cells and is a new model of virus-induced IDDM. To investigate genetic susceptibility to KRV-induced diabetes, major histocompatibility complex congenic and other inbred rats were infected with the virus and studied for the appearance of diabetes and insulitis. KRV infection alone induced insulitis, selective β-cell necrosis, and diabetes in BB/Wor DR and LEW1.WR1 (RT1 Au B/Du Ca) but not other rats. Thus, KRV, an environmentally ubiquitous rat parvovirus, can precipitate autoimmune diabetes in rats that are not susceptible to spontaneous diabetes. If rats are injected with poly(I·C) immediately before KRV infection, diabetes frequency increases to >90% in BB/Wor DR and LEW1.WR1 rats, and PVG.RT1u rats are converted from KRV-resistant to KRV-susceptible status. Susceptibility to KRV-induced diabetes thus requires the presence of class I Au and class II B/Du gene products, which are shared by DR, LEW1.WR1, and PVG.RT1u rats. The RT1u haplotype is not sufficient for susceptibility, however, because while WF rats are RT1u, they resist KRV-induced diabetes. If rats are depleted of RT6.1+ regulatory T-cells before KRV infection, the frequency of diabetes is dramatically increased in DR and LEW1.WR1, but not PVG.RT1u or other rats. These data confirm a regulatory role of RT6.1+ T-cells in diabetes induction, but indicate that they may not operate as such in all rat strains. KRV-induced diabetes is T-cell–mediated: DR and LEW1.WR1 rats are protected from diabetes by treatment with monoclonal antibodies directed against αβ T-cell receptor (TCR)+, CD5+, and CD8+ T-cells. Concanavalin A–activated spleen cells from KRV-infected DR rats adoptively transfer diabetes and insulitis into class IIu compatible rats, suggesting that KRV infection of susceptible rats leads to the activation of diabetogenic class IIu restricted T-cells. The ability of a common rat virus to initiate IDDM in multiple strains of rats strengthens the possibility that viruses may also initiate IDDM in human populations.