Detection of primary colorectal cancer with indium 111 monoclonal antibody B72.3. B72.3 is a murine monoclonal antibody of the immunoglobulin subclass IgG1 directed against TAG-72, a cell surface antigen present on colorectal carcinoma cells. We investigated the utility of scanning with indium 111-labeled B72.3 in 16 patients with a high clinical suspicion of or biopsy-proven primary colorectal cancer. Each patient received 1 or 2 mg of B72.3 monoclonal antibody labeled with 152 MBq of indium 111. Patients underwent scanning 2 to 3 days and 7 days after infusion by planar and emission computed tomography. Nineteen lesions were confirmed in 12 patients. Three patients with benign polyps had true-negative monoclonal antibody scans. Indium 111-labeled imaging of B72.3 detected nine of 19 lesions. Unsuspected tumor sites were identified by monoclonal antibody scan in three patients. By detection of additional abdominal disease and extra-abdominal spread, indium 111-labeled scanning of B72.3 directly affected treatment in 18% of patients. 