The indirect fluorescence method has been used to detect and localize carcinoembryonic antigen (CEA) in specimens of colonic carcinoma, other malignant or benign tumours, and non-cancerous tissues. The antigen was found in 39 specimens of colonic or rectal adenocarcinomas with a characteristic location at the apical pole of the cancerous cells. On the other hand, CEA was not detectable in two samples of anaplastic colonic carcinomas. Furthermore, CEA was occasionally found in some other digestive or non-digestive carcinomas. It was also present in 19 benign colo-rectal polyps examined and in the majority of 85 morphologically non-malignant colonic glands obtained from specimens excised for cancerous or non-cancerous diseases.