A monoclonal antibody (MoAb) directed against the human epidermal growth factor receptor 2 (HER2; ERBB2) labeled with iodine I 131 using the residualizing radio-iodinating reagent N-succinimidyl 4-guanidinomethyl 3-iodobenzoate (SGMIB), with potential radiotherapeutic and radioimaging activities upon positron emission tomography (PET). Upon administration of iodine I 131 SGMIB-anti-HER2 VHH1, the HER2 MoAb moiety selectively targets and binds to HER2-expressing tumor cells. Upon PET imaging, tumor cells can be visualized. In addition, the iodine I 131 moiety of VHH1 selectively delivers a cytotoxic dose of iodine I 131 to the tumor cells. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types. SGMIB improves tumor retention of radioactivity and decreases exocytosis.