α-Glucan lyase was purified from a seaweed, Graciralia chorda, by means of starch adsorption, ion-exchange chromatography and gel-filtration. The enzyme was stabilized and activated in the presence of Ca²⁺ and CI^- ions. Chemical modifications showed that a carboxyl residue was essen-tial to the activity and a tryptophanyl residue(s) was probably involved in substrate binding. The enzyme degraded waxy rice amylopectin from its non-reducing ends and left the outermost chains as glucosyl residues. 1, 5-Anhydro-D-arabino-hex-2-ulose (1, 5-anhydro-D-fructose) was prepared by reacting the enzyme on waxy corn starch with 50% yields. The sugar served as an antioxidant on the oxidation of linoleic acid and its ability was -2-fold as high as that of ascorbic acid.