Two kinds of complement activating (anti-complementary) polysaccharides, which were expected to be immunomodulators were purified from Arecae Pericarpium (the pericarps of Areca catechu), and their action modes have been studied. The active polysaccharides, AC-2-IIIa and AC-2-IIIc from Arecae Pericarpium showed dose-dependent anti-complementary activities on TCH_(50). The anti-complementary activities of AC-2-IIIa and AC-2-IIIc in metal ion-free condition were completely decreased in comparison with control whereas in case of Ca^(2+)-free condition, these activities were maintained, considerably. Also AC-2IIIa and AC-2-IIIc showed relatively potent alternative complement pathway activities. Furthermore, after incubation of the normal human serum with polysaccharide of Arecae Pericarpium in the absence of Ca^(2+) ion, a cleavage of C3 in the serum was found to have occurred through immunoelectrophoresis (IEP) with anti-human C3. Also, from the results of IEP using anti-human whole serum, the ratios of the height of 3rd peak to α2-M peak by AC-2-IIIa and AC-2-IIIc proved to be 1.50±0.04 and 1.22±0.08, respectively. These results indicate that the modes of complement activation by AC-2-IIIa and AC-2-IIIc from Arecae Pericarpium are via both the classical pathway and the alternative pathway.