Ulex europaeus agglutinin-1 (UEA-1) binds specifically to alpha-L-fucose, the terminal sugar of blood group substances H2 and Ley the expression of which is confined mainly to the proximal colon. The aim of the study was to determine whether there is neo-expression of alpha-L-fucose in the normal appearing mucosa of the distal bowel in subjects with or at risk of hereditary bowel cancer. UEA-1 binding was investigated by means of a direct lectin-peroxidase technique in tissues derived from patients with familial adenomatous polyposis (9) and affected (16) or at risk of (14) hereditary non-polyposis colorectal cancer. Control tissues were obtained from normal subjects and patients with sporadic bowel cancer and no family history of the condition. There was increased expression of alpha-L-fucose in some adenomas (mosaic pattern), in most carcinomas (diffuse), in hyperplastic polyps (crypt base), but not in normal appearing mucosa or transitional mucosa (adjacent to carcinoma) in any of the groups of patients examined. In view of the negative findings for normal-appearing mucosa, the suggestion that there is a pre-neoplastic field change in subjects at risk of hereditary bowel cancer may be unfounded.