Insulin-dependent diabetes is an autoimmune disease targeting pancreatic β-islet cells. Recent data suggest that autoreactive CD8+ T cells are involved in both the early events leading to insulitis and the late destructive phase resulting in diabetes. Although therapeutic injection of protein and synthetic peptides corresponding to CD4+ T cell epitopes has been shown to prevent or block autoimmune disease in several models, down-regulation of an ongoing CD8+ T cell-mediated autoimmune response using this approach has not yet been reported. Using CL4-TCR single transgenic mice, in which most CD8+ T cells express a TCR specific for the influenza virus hemagglutinin HA 512–520 peptide: K d complex, we first show that iv injection of soluble HA 512–520 peptide induces transient activation followed by apoptosis of Tc1-like CD8+ T cells. We next tested a similar tolerance induction strategy in (CL4-TCR× Ins-HA) F 1 double transgenic mice that also express HA in the β-islet cells and, as a result, spontaneously develop a juvenile onset and lethal diabetes. Soluble HA 512–520 peptide treatment, at a time when pathogenic CD8+ T cells have already infiltrated the pancreas, very significantly prolongs survival of the double transgenic pups. In addition, we found that Ag administration eliminates CD8+ T cell infiltrates from the pancreas without histological evidence of bystander damage. Our data indicate that agonist peptide can down-regulate an autoimmune reaction mediated by CD8+ T cells in vivo and block disease progression. Thus, in addition to autoreactive CD4+ T cells, CD8+ T cells may constitute targets for Ag-specific therapy in autoimmune diseases.