Abstract Two glucans (PONGa and PONGb) differing in their anomeric and glycosidic linkage structures were isolated from the water-insoluble materials (PON) of Pleurotus ostreatus basidiocarps, which activated the complement system and were almost soley composed of D-glucose. The isolatIon was achieved by repeated precipitations with ethanol and adsorption on concanavalin A (Con A) of paN suspension in thymol/NaCL Based on methylation analysis. IR, GLC-MS, $^1H,{;}and{;}^{13}C-NMR$ spectroscopies, PONGa was found to be a branched a-glucan composed of ${alpha}-linked$ D-glucopyranose residues and ${alpha}-linked$ units with 6-branching points, whereas PONGb was a linear ${eta}-1,3-glucan$ composed mainly of ${eta}-1,3-linked$ D-glucopyranose residues. The PONGb particles reacted more potently than the PONGa particles as C3 activator in alternative complement hemolysis and crossed-immunoelectrophoresis using anti-human C3, thereby suggesting that the complement activating components of PON were ${eta}-(13)-glucans rather$ than ${alpha}-glucan$ components.onents.