A novel glucan, called cnidirhan SI, was isolated from the rhizome of Cnidium officinale MAKINO. It was homogeneous on electrophoresis and gel chromatography, and its molecular mass was estimated to be 1.3×10⁴. It is composed of D-glucose alone, in addition to small numbers of O-acetyl groups. Methylation analysis, nuclear magnetic resonance and enzymic degradation studies indicated it has a high-branched glucan type structure composed of α-1, 4-linked D-glucopyranose residues with both 3, 4- and 4, 6-branching points. The glucan showed significant reticuloendothelial system-potentiating activity in a carbon clearance test, as well as pronounced anti-complementary activity. This substance is the first example of a branched α-glucan with phagocytosis-stimulating and anti-complementary activities.