Chemically induced tumors of inbred mice elicit immunity in animals in which the tumors are induced and in other animals of the same inbred stock. The immunity is specific for each tumor: even two tumors induced in one animal with the same carcinogen are not cross-reactive. Immunity to cancer has since been observed in the case of sarcomas and carcinomas induced by a number of chemical and physical carcinogens and in several species, including mice, rats, and guinea pigs. The nature of molecules which mediate immunity to tumors is a central question in cancer immunology. A small number of such molecules have been biochemically defined. Of these, some are viral antigens expressed in tumor cells, while the relationship of some others to viral antigens is unclear. A surprising majority of nonviral tumor antigens have turned out to bear homology with stress-induced proteins. Four families of such molecules are discussed: the gp96 (hsp100) and p84/86 (hsp90) antigens of chemically induced mouse sarcomas, hsp70 antigens of tumors obtained by transfection of normal rat fetal fibroblasts with an H-ras oncogene, and the albuminoid antigens of murine melanomas and a rat histiocytoma. (Albumin-like antigens are included among the stress-induced proteins because albumin, though constitutively expressed in adult tissues, is heat shock inducible in fetal liver.) Each of these antigens is a moderately abundant protein, present not only in tumors but also in normal tissues. Administration of each of these antigen preparations from the tumor, but not from normal tissue, renders the animal immune to challenge with live cells of the tumor from which the antigens are prepared. And yet, no structural differences in the antigens have been observed between normal tissues and tumors. It is suggested that these stress-induced proteins may not be tumor antigens per se, but may be carriers of immunogenic moieties such as short peptides. The stress-induced proteins may therefore serve either as antigen-presenting molecules like the MHC-encoded molecules or as accessory molecules in the presentation of antigens by MHC molecules. The ability of stress-induced proteins to bind to a variety of molecules, including peptides, is consistent with this possibility.