Genetically modified mice with spontaneous development of mammary carcinoma provide a powerful tool to study the efficacy of tumor vaccines, since they mimic breast cancer development in humans We used a transgenic murine model expressing poly-omavirus middle T oncogene and mucin I tumor-associated Ag to determine the prevenfive effect of a dendrific/tumor fusion cell vaccine. The MMT (a transgenic murine model) mice developed mammary carcinoma between the ages of 65-108 days with 100% penetrance. No spontaneous CTL were detected. However, prophylacfic vaccinafion of MMT mice with dendrific/tumor fusion cells induced polyclonal CTL activity against spontaneous mammary carcinoma cells and rendered 57-61% of the mice free of the disease at the end of experiment (180 days). Furthermore, the level of CTL activity was maintained with mulfiple vaccinafions. The antitumor immunity induced by vaccination with dendrific/tumor fusion cells reacted differently to injected tumor cells and autochthonous tumor. Whereas the injected tumor cells were rejected, the autochthonous tumor evaded the atiack and was allowed to grow. Collecfively these results indicate that prophylactic vaccinafion with dendrific/tumor fusion cells confers sufficient antfiumor immunity to counter the tumorigenesis of potent oncogenic products The findings in the present study are highly relevant to cancers in humans.