It is proposed that T cell nonresponsiveness to H-Y is attributable to natural tolerance, maintained in the periphery by an anti-idiotype suppressor mechanism. Induction of such a suppressor mechanism occurs when H-Y plus certain K or D antigens mimic other self-alleles of K or D antigens either alone or in conjunction with minor histocompatibility antigens. The application of the model to other systems, as well as predictions made from it are discussed.