The need to modify tumor cells in order to render them more "immunogenic" was based on the assumption that normal, nonmodified tumor cells are non- or weakly immunogenic and as such are unable to raise an efficient protective immune response. Various methods for "xenogenization" (modification of tumor cells) were suggested: induction of new foreign antigens, treatment with either chemicals or enzymes and use of mutagens. Xenogenized tumor cells by their coupling to proteins, and use of chemicals like DTIC (5-[3,3-dimethyl- 1-triazeno]-imidazole-4-carboxamide), TZC (8-carbamoyl-3-methyl-imidazo[5, 1-d]- 1,2,3,5-tetrazin-4 [3H]-one 8-carbamoyl-3-[2-chloroethyl] imidazole [5,1 -d]- 1,2,3,5-tetrazin-4[3H]-one) and antiemetic drugs, were tested in experimental models of murine leukemia. Non-tumorigenic clones, xenogenization with DNA hypomethylating agents, aryl-triazine derivatives and DTIC were evaluated for their induction of protective immune response in murine lymphoma. Murine plasmacytoma cells were used for immunization after treatment with glutaraldehyde. Viral modifications of tumor cells were evaluated for their ability to induce a protective tumor response in model systems of rat fibrosarcoma, liver metastatic rat tumor cells, lymphoid tumor cells and hamster tumor cells. In the case of human cancer, attempts were reported to use DNP-conjugated melanoma cells, mutagenic triazine compounds, an autologous colon tumor cell bacillus Calmette-Guerin (BCG) vaccine and genetically engineered vaccines for immunization. The general conclusion drawn from experimental tumor models and for human cancer is, that although modified tumor cells were found to be partially effective in experimental models, it is still necessary to provide more data in order to determine the effective use of xenogenized human tumor cells for immunotherapy.