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Structural requirements for antifreeze and antilectin activities of the antifreeze glycoproteins from the Antarctic fish Trematomus borchgrevinkihave been compared. Graded removal of different amounts of the disaccharide side chain by β elimination caused similar reductions in the capacities for depressing the freezing temperature of water (antifreeze activity) and for inhibiting hemagglutination by Osageorange lectin (antilectin activity). Both activities were also reduced to nearly similar extents by acetylation of hydroxyls of carbohydrate residues, positioning a negative charge on the C-6 carbons of the carbohydrate, or by formation of borate complexes. But oxidations of the C-6 hydroxyls of the carbohydrate to aldehyde groups caused no reductions of either activity. Structural similarities required for both activities therefore exist.