A novel β-heteroglucan, called cnidirhan SIIA, was isolated from the rhizome of Cnidium officinale MAKINO. It was homogeneous on electrophoresis and gel chromatography, and its molecular mass was more than 1×10⁷. It was composed of D-glucose, D-galactose and L-arabinose in the molar ratio of 85 : 5 : 8, in addition to small numbers of O-acetyl groups. Methylation analysis and nuclear magnetic resonance studies indicated its main structural features composed of β-1, 6-linked D-glucopyranosyl residues with both 3, 6- and 4, 6-branching points. In addition, it has both β-1, 4-linked L-arabinopyranosyl and β-1, 6-linked D-galactopyranosyl residues. The polysaccharide showed very pronounced reticuloendothelial system-potentiating activity in a carbon clearance test, as well as a marked anti-complementary activity.