An indium I 111-labeled trastuzumab with potential use as an imaging agent. Indium In 111 CHX-A DTPA trastuzumab is chemically conjugated via a bifunctional metal chelator molecule, 2-(p-isothiocyanatobenzyl)-cyclohexyl-diethylenetriaminepentaacetic acid (CHX-A DTPA), a backbone-substituted derivative of DTPA. This agent may allow radioimmunolocalization of HER2-positive cells. Trastuzumab, a recombinant humanized monoclonal antibody that selectively binds with high affinity to the extracellular domain of human epidermal growth factor receptor 2 (HER2), may elicit an antibody-dependent cellular cytotoxicity (ADCC) against tumor cells that overexpress HER2.