Two kinds of complement system activating polysacchardies, which were expected to be immunomodulators were purified from the pericarp of Areca catechu, and their action modes have been studied. AC-2-IIIa and AC-2-IIIc from A. catechu showed dose-dependent anti-complementary activity on TCH_(50). The anti-complementary activities of AC-2-IIIa and AC-2-IIIc in metal ion free condition (EDTA-GVB^(--)) were completely decreased in comparison with control whereas in case of Ca^(++) free condition (Mg^(++)-EGTA-GVB^(--)) these activities caused by polysaccharides were maintained considerably. Also AC-2-IIIa and AC-2-IIIc were incubated with NHS in Mg^(++)-EGTA-GVB^(--), and then a hemolytic assay (ACH_(50)) were carried out using fresh rabbit erythrocytes. These polysaccharides considerably inhibited ACH_(50). Furthermore after incubation of the normal human serum with purified polysaccharide of A. calechu in the absence of Ca^(++) ion, a cleavage of C3 in the serum was found to have occurred through immuno-electrophoresis using goat anti-human C3 seum. These results indicate that the mode of complement activation by AC-2-IIIa and AC-2-IIIc from A. catechu is via both the alternative and classical pathway.