Water-soluble and alkaline-soluble crude polysaccharides which were separated from the roots or leaves of Panax ginseng C. A. Meyer, were compared for their anti-ulcer activity. Of these four polysaccharide fractions, the water-soluble crude polysaccharide fraction (GL-2) from the leaves and the alkaline-soluble crude polysaccharide fraction (GRA-2) from the roots prevented HCl/ethanol-induced ulcerogenesis in mice potently. The most potent fraction, GL-2, was further fractionated into four polysaccharide fractions by precipitation with cethyltrimethylammonium bromide, and the weakly acidic polysaccharide fraction, GL-4, showed the most potent inhibition of gastric lesion formation. The activity of GL-4 decreased after treatment with periodate or digestion with endo-polygalacturonase, indicating that the carbohydrate moiety may contribute to the expression of the activity. GL-4 was further purified by anion-exchange chromatography and gel filtration, and the most active purified polysaccharide, GL-4IIb1III was obtained. GL-4IIb1III (average relative molecular mass, 16,000 d) had the nature of a pectic polysaccharide, and was composed mainly of galactose and galacturonic acid with small proportions of rhamnose, arabinose, mannose, glucose, and glucuronic acid. GL-4IIIb1III prevented HCl/ethanol-induced ulcerogenesis in mice dose dependently.