Tumor-specific transplantation antigens are antigens that can lead to complete immunological destruction of a transplanted cancer by the syngeneic host. When such antigens are expressed on cancers induced by chemical or physical carcinogens, then they are usually unique, i.e., antigenically different for each independently induced tumor. In this study, we show that the product of a gene encoding a novel MHC class I molecule and isolated from the murine UV light-induced regressor tumor 1591 represents one such unique tumor-specific transplantation antigen that causes tumor rejection. The major evidence comes from our finding that 1591 progressor variants regularly lost the gene encoding this antigen that is expressed in the parental tumor that regresses in normal mice; furthermore, reintroduction of this gene into a 1591 progressor variant by DNA transfection caused the progressor variant to regress in normal immunocompetent mice. Thus, the progressor tumor reverted to the parental regressor phenotype following transfection. Consistent with the conclusion that the expression of the novel MHC class I gene following transfection was responsible for the regressor phenotype is also our finding that a variant of the transfected tumor that had lost expression of the transfected gene resumed its progressive growth behavior. Finally, we show that the molecule encoded by the novel class I gene is specifically recognized by a syngeneic tumor-specific cytolytic T cell clone that we have previously shown to select in vitro for progressor variants from the parental regressor tumor cell line. It remains to be determined to what extent unique tumor-specific rejection antigens of other highly immunogenic regressor tumors are encoded by novel MHC class I genes and whether these genes represent germline mutations or somatic mutations caused by the carcinogen treatment.