Metastatic clones of some tumors manifest an impaired expression of class I major histocompatibility complex (MHC) antigens. High metastatic, low immunogenic Lewis lung carcinoma clones (C57BL-H-2b origin) express low levels of the H-2Kb MHC antigen. These cells metastasize spontaneously in C57BL/6J mice. Transfection of syngeneic or allogeneic H-2K genes converted such cells to the nonmetastatic state, but did not prevent the growth of the local tumors. Transfection of two H-2K genes, syngeneic and allogeneic, into the highly metastatic clone D122, resulted in reduction of the growth rates of the transfectants and protected the mice from D122 metastases. In contrast, cells transfected with a single class I gene (syngeneic or allogeneic) gave partial protection, or did not protect the mice at all from D122 metastases. The combination of syngeneic and allogeneic genes in the same tumor cell elevated the immunogenic properties of the expressing cells and potentiated the immune response as was demonstrated by in vitro cytotoxicity analysis and by limiting dilution cytotoxicity analysis. Increased immunogenicity by double transfection may constitute an effective therapeutic modality.