An acidic polysaccharide, called peonan PA, was isolated from the root of Paeonia lactiflora PALLAS. It was homogeneous on electrophoresis and gel chromatography, and its molecular mass was estimated to be 6.0×10⁴. Peonan PA is composed of L-arabinose : D-galactose : D-galacturonic acid in the molar ratio of 2 : 1 : 10, in addition to small amounts of O-acetyl groups and peptide moieties. About forty percent of the hexuronic acid residues in peonan PA exist as methyl esters. Reduction of carboxyl groups, methylation analysis, lithium degradation and nuclear magnetic resonance studies indicated that its main structural features involve both α-1, 5-linked L-arabino-β-3, 6-branched D-galactan type and α-1, 4-linked D-galacturonan type structural units. The polysaccharide exhibited remarkable reticuloendothelial system-potentiating activity in a carbon clearance test and considerable anti-complementary activity.