&NA;Thirty patients who had curative surgery for sigmoid and rectal carcinoma were studied by immunoscintigraphy using a new monoclonal antibody: B72.3 radiolabeled with Iodine‐131 (I‐131) or Indium‐111 (In‐111). Results were compared with other conventional diagnostic procedures such as computed tomography, ultrasound, endoscopy, and tumor markers. Seven patients had local recurrences and/or distant metastases. Immunoscintigraphy identified all local recurrences (five cases) that were confirmed by CT and was able to localize three of five liver metastases (two false‐negatives). Furthermore, immunoscintigraphy also recorded a false‐positive. Preliminary use of B72.3 shows good potential for the evaluation and detection of neoplastic sites.