A 3-layer immunoperoxidase technique was used to demonstrate carcinoembryonic antigen (CEA) in colonic polyps from patients with or without previous or concurrent malignancy. CEA was demonstrated in a higher percentage of the polyps received as fresh specimens that were rapidly frozen and fixed in ethanol, than in formalin-fixed, paraffin-embedded sections. Tissue CEA content of both colonic carcinomas and polyps was determined by radioimmunoassay, and it was found that benign colonic tumours had levels of tissue CEA comparable to colonic cancer, indicating that CEA concentration in a tumour does not reflect its grade of malignancy. In fact, in one case in which both colonic cancer and polyps were removed, the polyps had the higher quantities of tissue CEA. Further, tissue CEA concentration of a polyp was not dependent on its size or location. Studying the titres of circulating CEA in these patients revealed an elevation of plasma CEA in one-third of the patients with only colonic polyps, whilst the patients with cancer all had increased titres.