NOT all peripheral tissue antigens enter the thymus and it is unclear how the immune system remains tolerant to this class of self antigen. As tolerance to self peptides can generate gaps in the T-cell repertoire for cross-reactive foreign antigens^1,2, we investigated whether this mechanism might also diminish autoimmune reactions to similar peptides expressed by peripheral tissues. Herpes stromal keratitis (HSK) is a virally induced autoimmune reaction against corneal tissues mediated by T cells^3á¤-5, and is a leading cause of human blindness⁶. Resistance to HSK in mice is associated with allotypic variation in immunoglobulin genes^7,8, possibly because circulating immunoglobin-derived peptides can cross-tolerize T cells specific for corneal tissue autoantigens. Here we show that HSK is mediated by T-cell clones specific for corneal self antigens which also recognize an allotype-bearing peptide derived from IgG2a, and that exposure of HSK-susceptible mice to a soluble form of this peptide confers resistance to HSK. Shared expression of peptide subsequences between sequestered tissue proteins and circulating proteins may be important for maintenance of self-tolerance and prevention of autoimmunity.