In vivo studies have shown that regulatory CD4(+) T cells regulate conventional CD4(+) T cell responses to self- and environmental Ags. However, it remains unclear whether regulatory CD4(+) T cells control CD8(+) T cell responses to self, directly, or indirectly by decreasing available CD4(+) T cell help. We have developed an experimental mouse model in which suppressive and helper T cells cannot mediate their functions. The mouse chimeras generated were not viable and rapidly developed multiple organ autoimmunity. These features were correlated with strong CD8(+) T cell activation and accumulation in both lymphoid and nonlymphoid organs. In vivo Ab treatment and secondary transfer experiments demonstrated that regulatory CD4(+) T cells play an important direct role in the prevention of peripheral CD8(+) T cell-mediated autoimmunity.