Immune regulation plays an important role in the establishment and maintenance of self-tolerance. Nevertheless, it has been difficult to conclude whether regulation is Ag specific because studies have focused on polyclonal populations of regulatory T cells. We have used in this study a murine transgenic model that generates self-reactive, regulatory T cells of known Ag specificity to determine their capacity to suppress naive T cells specific for other Ags. We show that these regulatory cells can regulate the responses of naive T cells with the same TCR specificity, but do not inhibit T cell proliferation or differentiation of naive T cells specific for other Ags. These results demonstrate that immune regulation may be more Ag specific than previously proposed.