Two-step radioimmunotargeting using a bispecific anti-CEA/anti-in-DTPA monoclonal antibody and an 111In-labeled DTPA dimer (diDTPA-TL) was evaluated nine times in eight patients with medullary thyroid cancer (MTC). Immunoscintigraphy was performed 5 and 24 hr after injection of 111In-diDTPA-TL. For five patients, radioimmunoguided surgery (RIGS) was performed using a hand-held gamma probe (sodium iodine), and a biodistribution study was performed 48 hr (four times) and 24 hr (one time) after injection of 111In-diDTPA-TL. Mean tumor uptake (%ID/kg in tumor) was 39 (range 2.75-139). In these five patients, immunoscintigraphy visualized all known tumors and detected unknown foci (US and CT were negative) in the neck (once) and neck and liver (once). Immunoscintigraphy, performed four times in search of a recurrence, detected unknown localizations in the mediastinum and neck (twice) and was negative twice. There were no false-positives. In three of five patients who had surgery, RIGS localized tumor foci not detected by the surgeon. RIGS failed to detect two small lesions (10 x 10 mm) corresponding to sites of fibrosis and microscopic cancer infiltration. Bispecific anti-CEA/anti-In-DTPA mediated targeting of 111In-diDTPA-TL provided elevated tumor uptake and tumor-to-normal tissue ratios. Radioimmunodetection of small MTC lesions is thus possible even when morphological imaging techniques prove negative.