Two polysaccharides, called glycyrrhizans UA and UB, were isolated from the root of Glycyrrhiza uralensis FISCHER. They were homogeneous on electrophoresis and gel chromatography, and showed reticuloendothelial system-potentiating activity in a carbon clearance test. Glycyrrhizan UA is composed of L-arabinose : D-galactose : L-rhamnose : D-galacturonic acid in the molar ratio of 20 : 14 : 1 : 3, and glycyrrhizan UB is composed of L-arabinose : D-galactose : D-glucose : L-rhamnose : D-galacturonic acid in the molar ratio of 12 : 10 : 1 : 10 : 20, in addition to small amounts of O-acetyl groups and peptide moiety, respectively. About 10% (glycyrrhizan UA) and 35% (glycyrrhizan UB) of the D-galacturonic acid residues exist as the methyl esters. Methylation analysis, carbon-13 nuclear magnetic resonance and periodate oxidation studies indicated their structural features.