The HER-2/neu receptor is a member of the epidermal growth factor family and is amplified in multiple cancers. It is under intense investigation both as a prognostic marker and for therapy, using monoclonal antibodies targeted against the receptor. We have developed a novel two-component gadolinium-based MR contrast agent to image the HER-2/neu receptor. Positive T1 contrast in MR images was generated by the specific binding of avidin-gadolinium complexes to tumor cells prelabeled with a biotinylated anti-HER-2/neu antibody. Significant intensity enhancement was observed in HER-2/neu-expressing cell lines and in vivo in a breast cancer model. Potential applications of this approach may include determination of the HER-2/neu status for prognosis and for selecting tumors for monoclonal antibody therapy.