The rhizomes of Acorus gramineus have frequently been used in traditional medicine mainly for sedation as well as enhancing brain function. In this study, the anti-allergic activity of A. gramineus was investigated. The 70% ethanol extract of the rhizomes of A. gramineus was found to inhibit the allergic response against 5-lipoxygenase (5-LOX)-catalyzed leukotriene (LT) production from rat basophilic leukemia (RBL)-1 cells and b-hexosaminidase release from RBL-2H3 cells with IC50`s of 48.9 and >200 mg/ml, respectively. Among the 9 major constituents isolated, b-asarone, (2R,3R,4S,5S)-2,4-dimethyl-1,3-bis (2`,4`,5`-trimethoxyphenyl)tetrahydrofuran (AF) and 2,3-dihydro-4,5,7-trimethoxy-1-ethyl-2-methyl-3-(2,4,5-trimethoxyphenyl)indene (AI) strongly inhibited 5-LOX-catalyzed LT production in A23187-treated RBL-1 cells, AI being the most potent (IC50=6.7mM). Against b-hexosaminidase release by antigen-stimulated RBL-2H3 cells, only AI exhibited strong inhibition (IC50=7.3mM) while b-asarone and AF showed 26.0% and 39.9% inhibition at 50mM, respectively. In addition, the ethanol extract of A. gramineus showed significant inhibitory action against the hapten-induced delayed hypersensitivity reaction in mice by oral administration at 200 mg/kg. Therefore, it is suggested that A. gramineus possesses anti-allergic activity and the constituents including b-asarone and AI certainly contribute to the anti-allergic activity of the rhizomes of A. gramineus.