Abstract: The disaccharide .beta.-D-Gal (1->>3)-D-GalNAc, which specifically binds with peanut agglutinin (PNA) or oxidized by galactose oxidase, has been discovered in the colorectal mucus of patients with cancer or precancer. Because of the presence of .beta.-D-Gal-(1>>3)-D-GalNAc also on neuraminidase treated erythrocytes of the ABO type, their competitive binding with PNA has been exploited to develop a hemagglutination inhibition assay. Additional methods of simple detection of this disaccharide include a latex agglutination test, enzyme-avidin-biotinylated PNA, and a galactose oxidase strip test. This rapid, simple and inexpensive assay is designed to test the presence of .beta.-D-Gal-(1>>3)-D-GalNAc in large intestine mucus obtained by routine digital-rectal examination and has the potential for screening populations for large intestinal carcinomas.