Samples of tumor and normal mucosa from 32 patients with adenocarcinoma of the colorectum were examined for their capacity to bind radioiodinated antibody to carcinoembryonic antigen (anti-CEA) lgG. Twenty-three (72%) of the tumors bound significantly more antibody than the respective normal mucosa. The results indicate that radiolabeled anti-CEA may be useful in the in vivo localization of CEA-producing tumors and metastases in man, and may have application in vitro as a diagnostic marker of precancerous change in colorectal biopsies from patients at risk of developing colorectal cancer.