Immunogenic variants (Imm+) generated after the treatment of murine tumor cells with the mutagen N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) can produce a strong protective response against non-mutagenized parent tumor cells. The use of this methodology to treat human tumors is currently thwarted by technological difficulties in applying the findings obtained with murine models to human disease. Two of these difficulties are described in this study. The first is that Imm+ variants lose most of their immunogenicity after treatment with X-irradiation or mitomycin C. The second is that mutagen-treated tumor cells must be cloned so as to select for Imm+ variants, for the presence of as few as 0·001 per cent tumorigenic cells within the bulk population will result in the failure of the protective effect of the Imm+ variants. Because of these and other difficulties with mutagen-induced Imm+ variants, we have developed a different approach to producing such variants using transfection of tumor cells with foreign genes. In contrast to mutagen induced Imm+ variants, these variants have been shown to retain their immunogenicity after X-irradiation.