An acidic polysaccharide has been isolated from the seed of Plantago major L. var. asiatica D_ECAISNE. The polysaccharide, named plantasan, was homogenous on gel chromatography and glass-fiber paper electrophoresis, and it was the representative mucilage of the seed. The component carbohydrates of it were _D-xylose, _L-arabinose, D-galacturonic acid, L-rhamnose and D-galactose, and the molar ratio of them was 15 : 3 : 4 : 2 : O. 4.