The crude biopolymer (AS-S1) and endobiopolymer (AS-S2) were isolated from the dry stem bark of Acanthopanax sessiliflorus and tested for anti complement activity. The two potent anticomplement biopolymers, AS-1 and AS-2-Fr.I, were isolated by the combination of ion-exchange chromatography and gel filtration methods from the endo-biopolymers (AS-S2). The anticomplement activity of AS-1 (MW 12 kDa) and AS-2-Fr.I (MW 180 kDa) were found to be 84.4% and 100.0%, respectively, at the concentration of $25{mu}g/ml$. Activated pathway of the complement system occurred in both classical and alternative pathways, as evidenced by crossed immunoelectrophoresis(CIEP), where a major pathway was detected to be the classical one. It was found that the anticomplement activities of the periodate oxidized were decreased significantly, but those of pronase digested biopolymers of AS-1 and AS-2-Fr.I were decreased very little. The AS-1 contained 2,4,6-tri-O-methyl-D-glucitol, 2,3,6-tri-O-methyl-D-galacitol, and 2,3,6-tri-O-methyl-D-galacitol, which indicated that AS-1 contained a $(1{ ightarrow}3),;(1{ ightarrow}4)-linked$ glucopyranosyl residue and a $(1{ ightarrow}4)-linked$ galactosyl residue. AS-2-Fr.I contained mainly 2,4-di-O-methyl-D-mannitol and 2,3,4-tri-O-methyl-D-galacitol, which contained $(1{ ightarrow}3),;(1{ ightarrow}6)$ linked mannosyl and $(1{ ightarrow}6)$ linked galactosyl residues.