Monoclonal antibodies (mAbs) have been used to treat malignancies in humans with varying degrees of success. Progress has been hindered by the lack of suitable animal models, which would ideally consist of immunocompetent animals that are tolerant to tumor-associated antigens. Suitable models would allow the study and optimization of anti-tumor immunotherapy. We describe a murine model for the study of immunotherapy in colorectal cancers. Carcinoembryonic antigen (CEA) is a cell-surface glycoprotein that is expressed on normal human intestinal epithelium and that is overexpressed in intestinal tumors. Mice that are transgenic for the human CEA gene (CEA.Tg) were crossed with multiple intestinal neoplasia (MIN) mice. MIN mice carry a germline APC mutation and are prone to the development of intestinal adenomas. The offspring from the MIN x CEA.Tg cross developed intestinal adenomas that were shown by immunohistochemistry to overexpress CEA. Pharmacokinetic studies by using (125)I-labeled anti-CEA mAb PR1A3 showed rapid localization of antibody to tissues expressing CEA, especially the gastrointestinal tract. Macroscopic and microscopic radioautographic analysis of the gastrointestinal tracts from MIN/CEA.Tg mice indicated that PR1A3 targeted and was retained in tumors at levels higher than in areas of normal gut. These results demonstrate the utility of the MIN/CEA.Tg mouse as a model for the study of anti-CEA immunotherapy and, furthermore, demonstrate the efficiency of tumor localization by PR1A3.