The demonstration that functionally different T-cell subsets can be defined by the isoforms of the leukocyte-common antigen, CD45, that they express, has prompted studies on the roles of these subsets in autoimmunity. The results have led to the identification of a particular subset of CD4+ T cells that have the ability to inhibit autoimmune disease. Further, it has been shown that diabetes in the B-B rat can be transferred by in vitro activation of T cells by Staphylococcal enterotoxin suggesting that superantigens may play a role in the pathogenesis of this disease. However, in this system too, it appears that a subset of T cells can inhibit the induction of autoaggressive cells. In other experimental autoimmune diseases there is evidence that CD8+ T cells can be protective and that these cells may mediate this protection by the synthesis of transforming growth factor-beta.