The levels of plasma carcinoembryonic antigen (CEA) were estimated in 59 control subjects attending St Mark's Hospital and in 139 patients with uncomplicated ulcerative colitis. There was little difference in the CEA levels in the two groups. In the colitic patients, the actual CEA values could not be correlated with the age of the patient, the activity of the disease, the extent of bowel involvement, or the length of history. In addition, seven colitic patients with severe dysplastic changes in the rectal mucosa and seven patients with established carcinoma in colitis were studied. With one exception in each group, the CEA levels in these patients were within the range shown by the St Mark's Hospital control population.