Two acidic polysaccharides, called rehmannan SA and rehmannan SB, were isolated from the dried root of Rehmannia glutinosa LIBOSCHITZ. They were homogeneous on electrophoresis and gel chromatography, and their molecular masses were estimated to be 6.4×10⁴ and 7.9×10⁴, respectively. They were commonly composed of L-arabinose : D-galactose : L-rhamnose : D-galacturonic acid in the molar ratios of 10 : 10 : 1 : 1 (rehmannan SA) and 14 : 7 : 3 : 8 (rehmannan SB), in addition to small amounts of peptide moieties. About eighty percent (rehmannan SA) and about thirty percent (rehmannan SB) of the hexuronic acid residues exist as methyl esters. Reduction of carboxyl groups, methylation analysis, nuclear magnetic resonance and the controlled Smith degradation studies indicated the structural features of rehmannan SB, the major one in terms of the yield. It has a core structural unit of a characteristic rhamnose-rich arabino-3, 6-galactan type. Both polysaccharides showed remarkable reticuloendothelial system-potentiating activity in a carbon clearance test.