A radiolabeled divalent histamine-succinyl-glycine (HSG) hapten-peptide linked with the macrocyclic chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) to the beta-emitting radionuclide yttrium 90 (Y-90), with radioimmunotherapeutic activity. After pre-treating and targeting tumor cells with a bi-specific monoclonal antibody (BiMoAB) directed against both a tumor-associated antigen (TAA) and the HSG hapten-peptide, the HSG portion of the administered yttrium Y 90-DOTA-di-HSG peptide IMP-288 binds to the anti-HSG sequence on the BiMoAB. In turn, Y-90 delivers a cytotoxic dose of beta radiation to tumor cells expressing the specific TAA.