Depletion of the minor (∼10%) subpopulation of CD4⁺ T cells that co‐expresses CD25 (interleukin (IL)‐2 receptor α‐chain) by thymectomy of neonates on the third day of life or by treatment of adult CD4⁺ T cells with anti‐CD25 and complement results in the development of organ‐specific autoimmunity. Autoimmune disease can be prevented by reconstitution of the animals with CD4⁺ CD25⁺ cells. CD4⁺ CD25⁺‐mediated protection of autoimmune gastritis does not require the suppressor cytokines IL‐4, IL‐10, or transforming growth factor (TGF)‐β. Mice that express a transgenic T‐cell receptor (TCR) derived from a thymectomized newborn that recognizes the gastric parietal cell antigen H/K ATPase all develop severe autoimmune gastritis very early in life. CD4⁺ CD25⁺ T cells are also powerful suppressors of the activation of both CD4⁺ and CD8⁺ T cells in vitro. Suppression is mediated by a cell contact‐dependent, cytokine‐independent T–T interaction. Activation of CD4⁺ CD25⁺ via their TCR generates suppressor effector cells that are capable of non‐specifically suppressing the activation of any CD4⁺ or CD8⁺ T cell. Activation of suppressor effector function is independent of co‐stimulation mediated by CD28/CTLA‐4 interactions with CD80/CD86. We propose that CD4⁺ CD25⁺ T cells recognize organ‐specific antigens, are recruited to sites of autoimmune damage where they are activated by their target antigen, and then physically interact with autoreactive CD4⁺ or CD8⁺ effector cells to suppress the development of autoimmune disease.