The progression of autoimmune diabetes is regulated. We examined here the cellular controls exerted on disease that developed in the BDC2.5 T cell receptor-transgenic model. We found that all BDC2.5 mice with a monoclonal, beta cell-reactive, T cell repertoire developed diabetes before 4 weeks of age; transfer of splenocytes from young standard NOD (nonobese diabetic) mice into perinatal monoclonal BDC2.5 animals protected them from diabetes. The protective activity was generated by CD4+ alphabeta T cells, which operated for a short time at disease initiation, could be partitioned according to DX5 cell surface marker expression and split into two components. Protection did not involve clonal deletion or anergy of the autoreactive BDC2.5 cells, permitting their full activation and attack of pancreatic islets; rather, it tempered the aggressiveness of the insulitic lesion and the extent of beta cell destruction.