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 radionuclide gallium (Ga) 68 with potential use in diagnostic imaging. After pretargeting with a bispecific monoclonal antibody (BiMoAB) directed against both a tumor-associated antigen (TAA) and the HSG hapten-peptide, the HSG portion of administered gallium Ga 68-labeled di-HSG-DOTA peptide IMP-288 binds the anti-HSG portion of the BiMoAB; Ga-68 radioisotopic activity localized to tumor cells bearing the TAA can then be visualized upon positron emission tomography (PET).