A tri-Fab bispecific monoclonal antibody (BiMoAb) divalent for the carcinoembryonic antigen (CEA) and monovalent for histamine-succinyl-glycine (HSG) peptide-hapten. Anti-CEA/anti-HSG bispecific monoclonal antibody TF2 binds to the tumor associated antigen (TAA) CEA on CEA-expressing tumor cells. Subsequently, an HSG peptide-hapten carrying a radionuclide is administered, binding to the anti-HSG binding fragment on the BiMoAb. Depending on the characteristics of the radionuclide used, CEA-expressing tumor cells may then be radioimaged and/or treated radioimmunotherapeutically.