The tumor marker carcinoembryonic antigen (CEA) is a 180-kD glycoprotein expressed on epithelial cells of the gastrointestinal tract and respiratory organs. It is a member of a large family belonging to the immunoglobulin superfamily. In order to develop a mouse model to study tumor immunotherapy using CEA as a target antigen, we have established mice which are transgenic for the CEA gene. The human CEA transgene maintains its spatiotemporal expression pattern in transgenic mice thus providing a suitable model in which to optimize different immunotherapy approaches, with the advantage that negative side effects due to the presence of CEA in normal tissue can be analyzed.