Autoreactive T cells can be readily identified in the peripheral lymphocyte pool of both humans and experimental animals. Peripheral tolerance may be maintained by regulatory/suppressor T cells which prevent the activation of autoantigen-specific cells. Mice thymectomized on day 3 of life (d3Tx) develop a wide spectrum of organ-specific autoimmune diseases. Reconstitution of d3Tx mice with CD4+ CD25+ T cells from normal mice prevents the development of disease. Similarly, CD4+ CD25+ T cells prevent the transfer of disease by autoantigen-specific cloned T cells derived from d3Tx mice. Thus, regulatory T cells can prevent both the induction and effector function of autoreactive T cells. In vitro, the CD4+ CD25+ population is anergic to stimulation through the T cell receptor (TCR) and suppresses the proliferative responses of normal CD4+ CD25- cells by a contract-dependent mechanism. Suppression is not MHC-dependent, but requires activation of the CD4+ CD25+ population. The mechanism of suppression in vivo and the target antigen(s) of this unique regulatory population remain to be characterized.