A methanol extract (TMe-ext) from the dried rhizome of Alisma orientale was screened for anti-complemenetary activity in experimental models. In the animal models, it was found that TMe-ext inhibits zymosan-induced hind paw edema in rats and zymosan-activated rat serum (ZAS)-induced vascular permeability in mice. TMe-ext showed an inhibitory effect on complement-induced hemolysis through both the classical pathway and the alternative pathway. And TMe-ext inhibited hypotonic shock-induced hemolysis, but this effect was weak compared with the anti-complementary activities of TMe-ext. Four triterpenes (alisol A, alisol A monoacetate, alisol B and alisol B monoacetate) isolated from the rhizome also inhibited the complement-induced hemolysis through the classical pathway, but two sesquiterpenes (alismol and alismoxide) were ineffective. These results indicate that Alismatis Rhizoma shows anti-complementary activity, and its anti-complementary components are partially attributable to the terpene components mentioned above.