The majority of blood group substances from hog stomach linings and one of two blood group substances from human stomach linings were able to precipitate with the plant lectin, concanavalin A. However, none of the blood group preparations from human ovarian cyst fluids tested were precipitable. Concanavalin A precipitates with branched polysaccharides having terminal α-D-glucopyranosyl or α-D-mannopyranosyl residues and has a fairly wide specificity directed toward any terminal non-reducing sugar residues having the D-arabino configuration at C-3, C-4 and C-6 and an α-configuration at C-1. The isolation, by degradation of hog gastric mucin with NaOH-NaBH4, of an inhibitory disaccharide: 3 or 4-O-α-(2-acetamido-2-deoxy-D-glucopyranosyl)-D-galactitol (α-D-GNAc-(1 → 3 or 4)-D-galactitol) indicates the basis for the reaction of concanavalin A with hog gastric blood group substances. Two samples of hog gastric blood group substances were fractionated by concanavalin A on a preparative scale into two fractions: one had only blood group activity and was not precipitated by concanavalin A while the other fraction was precipitable by both anti-A and concanavalin A.