A polysaccharide--Chai-III-5311 had been isolated from hot water extract of Radix Bupleuri through ultrafiltration and dialysis. Chai-III-5311 was homogeneous with an average molecular weight of about 8000. Acid hydrolysis has proved that Chai-III-5311 is composed of galacturonic acid, galactose, glucose, arabinose, xylose, ribose, rhamnose and an unknown component.