Naturally arising CD25+ CD4+ regulatory T cells (TR) play an important role in the prevention of autoimmunity. TCR specificity is thought to play a critical role in TR development and function, but the repertoire and specificity of TR TCRs remain largely unknown. We find by sequencing of TRAV14 (Valpha2) TCRalpha chains associated with a transgenic TCRbeta chain that the TRand CD25- CD4+ TCR repertoires are similarly diverse, yet only partially overlapping. Retroviral expression of TCRalpha genes in TCR transgenic RAG-deficient T cells revealed that a high frequency of TCRs derived from CD25+ but not CD25- CD4+ T cells confers the ability to rapidly expand upon transfer into a lymphopenic host. Thus, these data show that a large proportion of naturally arising TR have substantially more efficient interactions with MHC class II bound peptides from the peripheral self than CD25- T cells.