The monoclonal antibody CEA 102 against carcinoembryonic antigen was produced by immunization with purified CEA, and used as a radiotracer for the imaging of colorectal cancer. CEA102 was labeled with 131I by the chloramine-T or the Iodogen method, and administered i.v. to 20 patients with liver metastases, local recurrences, and/or primary tumors. Planar scintigraphy was performed during 5 or 6 postinfusion days. Hypersensitivity reactions such as fever and chill occurred in 4 patients, but no side effects were noted after endotoxin was excluded. Overall sensitivity was 71.4% (primary tumor 4/4, liver metastasis 5/6, lymph node metastasis 1/1, local recurrence 5/10), and no false positive was observed. In the cases of local recurrences, this method was useful to distinguish postoperative fibrosis from local recurrence. These results revealed that radiolabeled CEA102 has a great potential in qualitative diagnosis of colorectal cancer.