Anti-idiotype antibodies (Ab2) that immunologically mimic tumor antigens are auspicious agents for the active immunization of cancer patients. We have developed W12, a rat monoclonal IgG1 Ab2 to MN-14, a murine anti-carcinoembryonic antigen (CEA) monoclonal antibody. W12 is specific for MN-14 and does not react with other isotype-matched anti-CEA monoclonal antibodies. Moreover, W12 inhibits the binding between MN-14 and CEA. Anti-CEA antibodies can be induced by immunization with W12 (but not with control rat IgG) in xenogenic animals (mice or rabbits). Immunoblotting studies indicate that the internal image determinant borne by W12 is conformational and requires the association of the heavy and light chains of the Ab2 molecule. This study indicates that W12 is a potential idiotype vaccine in patients with CEA-producing cancers.