A number of factors have been demonstrated to influence the induction of pathogenic autoimmune responses, including the loss of regulatory T cells. To assess the contribution of regulatory T cells in CD8(+) T cell-mediated autoimmunity, RIP-gp/P14 double-transgenic mice expressing the lymphocytic choriomeningitis virus (LCMV) glycoprotein (gp) on pancreatic beta-islet cells, together with T cells expressing an LCMV-gp-specific T cell receptor (TCR), were crossed to RAG 2-deficient mice. The loss of potentially regulatory T cells, however, did not contribute to diabetes induction. Surprisingly, both RIP-gp/P14-RAG(+/-) and RIP-gp/P14-RAG(-/-) developed spontaneous disease, suggesting an influence of the 129 genetic background on disease susceptibility. Further studies demonstrated that disease susceptibility was not due to nonspecific T cell activation, nor to enhanced cross-presentation of LCMV-gp, nor to decreased expression levels of the negative regulatory molecule CD5. Disease susceptibility did associate, however, with enhanced T cell responses. Thus, T cell hyperactivity combined with various genetic factors may predispose an individual to autoimmunity.