Peanut lectin is known to bind to B-D-Gal-(1 leads to 3)-D-GalNac which provides antigenic determination for the T (TAg) blood group antigen. We examined 33 rectosigmoid carcinomas and 15 corresponding controls for their ability to express peanut lectin-binding sites. In controls one could localize TAg to the supranuclear portion of the cell, however, in cancers one noticed a cytostructural relocalization of TAg with the following two major patterns: localization to the region of the glycocalyx and localization intracytoplasmically in the apical portion of the cell. These two patterns were associated with glandular differentiation. Less frequently noted or in association with the above was a mucin glob-like pattern and/or a fine diffuse intracytoplasmic pattern associated with solid, nonglandular areas. The more poorly differentiated cancers less frequently expressed peanut lectin-binding sites. Benign (nontransitional zone) epithelium in those patients whose tumor expressed TAg was negative for peanut lectin-binding sites in 66 per cent of the cases. Reduced tumoral glycosyltransferases may explain this increased synthesis of TAg in cancers as compared with controls, if one considers TAg to be an incomplete glycoprotein of the MN blood group system.