Indium-labeled anti-colorectal carcinoma monoclonal antibody accumulation in non-tumored tissue in patients with colorectal carcinoma. Indium-111- (111In) labeled murine monoclonal antibodies ZCE 025 (against carcinoembryonic antigen) and CYT-103 MAb B72.3 (against tumor-associated glycoprotein - 72) have been used to image patients with colorectal cancers with encouraging results. The objectives of this study were to assess the frequency and causes of 111In MAb localization in tumor-free, benign tissues. Thus, scans of 75 patients who have undergone exploratory surgery following radioimmunoscintigraphy with 111In-ZCE 025 (n = 37) or 111In-CYT-103 (n = 38) were reviewed in conjunction with operative and histopathology reports. Localization in non-tumored tissues was seen in 10.8% and 13.1%, respectively, of patients receiving ZCE 025 and CYT-103. The most common sites involved were: degenerative joint disease, abdominal aneurysms, postoperative bowel adhesions, and local inflammatory changes secondary to surgery or external irradiation. Review of patients' medical history and results of concurrent diagnostic modalities is likely to lessen the false-positive rate of 111In-labeled MAb scan interpretation. 