Seventeen water-soluble polysaccharides obtained from various plants were tested for anti-complementary activities. Considerable activity was observed for Zizyphus-arabinan [(l-5)-linked cY-L-arabinofuranosyl main-chain having (1+2)linked cY-L-arabinofuranosyl side-chains] from the dried fruit of Zizyphus j@ba var. inermis, paniculatan [highly branched, partially 0-acetylated, acidic mucous polysaccharide composed of (l-+2)-linked cY-r_-rhamnopyranosyl residues having branches composed of O-(4O-methyl-o-D-glucopyranosyluronic acid)-(1+4)-/3-Dgalactopyranose at position 4 and (1+4)-linked a-D-galactopyranosyluronic acid residues having /?-D-glUCOSylUrOUiC acid groups at position 31 from the inner bark of Hydrangea paniculutu, and Plantago-mucilage A [highly branched, partially Oacetylated, acidic mucous polysaccharide with a main chain of (l-+4)-linked PDxylopyranosyl residues having other /3-D-xylopyranosyl groups and branches composed of 0-(a-D-glucopyranosyluronic acid)-(1-3)~cr-L-arabinofuranosyl and O-(a-D-galactopyranosyluronic acid)-(l-,3)-a-L-arabinofuranosyl groups at position 3 as side chains] from the seed of Plantago asiatica. The majority of the mucous acidic polysaccharides from the plants belonging to Mulvuceae had weak activities. These polysaccharides commonly contained (l-+2) -a-t-rhamnopyranosyl-( 1-+4)-a-D-galactopyranosyluronic acid residues as the main chain. Glucomannans and pectin did not show any significant activity. The results of tests on C4 titration, anti-complementary activity in the absence of Ca*+, crossed immunoelectrophoresis, and activity of the alternative complement pathway using rabbit erythrocytes indicated that the mode of complement activation by paniculatan occurred mainly via the classical pathway, whereas activation by Zizyphus-arabinan and Plantago-mucilage A were both via the alternative and classical pathways. The mode of complement activation by paniculatan did not depend on its particulate state.