The immunogenicity of an antigenic cell is a distinctly different property from its antigenic composition. Two clinical problems might well be beneficially affected by modifying the immunogenicity of cell membrane antigens: Can the immunogenicity of tumor-specific antigens on malignant cells be increased so that effective immunity to the tumor will lead to its destruction? Conversely, can the immunogenicity of histocompatibility antigens on grafted organs be minimized so that they will survive a foreign host? Evidence is accumulating to indicate that under certain conditions a tumor vaccine can be developed utilizing autochthonous tumor cells, removed from the host, chemically modified in vitro, and reinjected into the original host which will effectively, augment specific immunologic defenses against residual tumor cells. Similarly, the moderately immunosuppressed host will tolerate grafts after suitable immunogenic modification in vitro. Both approaches may have ready clinical application, even before the mechanisms have been worked.