Carcinoembryonic antigen (CEA) is a tumor marker for the most common forms of adenocarcinomas. We have previously described C57BL/6 mice transgenic for the complete human CEA gene. Compared with humans, these mice reveal a conserved spatiotemporal CEA expression pattern. To establish animal models for CEA-targeted tumor immunotherapy, we have crossed CEA transgenic mice with mice that are genetically predisposed to tumor development. These immunocompetent animals should allow optimization of immunotherapy strategies for maximal destruction of tumor tissues with minimal damage to CEA-expressing normal tissues. To develop a breast tumor model, CEA transgenic mice were cross-bred with mice transgenic for the rat neu protooncogene controlled by the mouse mammary tumor virus long terminal repeat. Female offspring developed poorly differentiated breast tumors, none of which, however, expressed CEA. As a model for colorectal tumors, mice bearing a mutation in the Apc gene (Min mice) and the CEA transgene developed multiple intestinal adenomas with strong CEA expression in all tumor cells. CEA expression had no significant effect on tumor growth. Occasional, well-differentiated breast adenocarcinomas in female offspring expressed CEA focally in tumor cells lining pseudolumina. Cross-breeding Apc(Min/+) mice with neu transgenic mice did not reveal a synergistic effect on the kinetics of breast tumor formation. Finally, CEA transgenic mice crossbred with mice transgenic for the SV40 large T antigen regulated by the surfactant protein-C promoter, developed multiple lung adenocarcinomas that revealed a mosaic CEA expression pattern.