Glycoconjugates with terminal Galalpha1-3Galbeta1-4GlcNAc sequences (alpha-galactosyl epitopes, natural xenoreactive antigens) are present on various tissues in pigs and are recognized by human anti-alphagalactosyl (alphaGal) antibodies1. Hence xenotransplantation (pig-to-human) would trigger immune reactions involving complement activation and lead to the hyperactute rejection of the graft. Xenoreactive antigens are often studied by using the lectin Griffonia simplicifolia 1 isolectin B4 (GS1 B4), which shows high affinity to galactose. We here estimate the specificity of GS1 B4 for detecting various galactosyl epitopes by measuring lectin binding to neoglycoproteins, thyroglobulin and pig skeletal muscle. Enzyme linked lectin assays confirmed that GS1 B4 was highly specific to alpha-galactosylated neoglycoproteins while the lectin did not detect a beta-galactosylated ligand. The length of the sugar chains influenced the lectin-carbohydrate interaction. A monosaccharide linked to serum albumin showed higher lectin affinity than did neoglycoproteins with di- and tri-alpha-galactosyl epitopes. When the carbohydrate was extended, as in the xenoreactive pentasaccharide (Galalpha1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc), the carbohydrate- lectin interaction was meagre. Not only the terminal, but also the subterminal sugar affected the lectin binding because the GS1 B4 affinity to Galalpha1-3Gal was much stronger than to Galalpha1-3GalNAc. In bovine and porcine thyroglobulin most alphaGal epitopes appear to be cryptic, but are unmasked by a heat denaturation. In pig skeletal muscle there was lectin reaction not only in the muscle capillaries, but also in the connective tissue and intracellularly in muscle fibres. In Western blots of isolated proteins from pig muscle at least three bands were strongly stained after incubation with lectin.