Some anti-allergic activities of the rhizome of Atractylodes japonica have been reported previously. But, the active principle(s) for anti-allergic action is not fully elucidated. In the present study, the 70% ethanol extract of the rhizome of A. japonica was prepared. For elucidation of its anti-allergic activity, the inhibitory activity against 5-lioxygenase (5-LOX)- catalyzed leukotriene (LT) production from rat basophilic leukemia (RBL)-1 cells and s-hexosaminidase release from antigen-treated RBL-2H3 cells was examined. And the extract was found to significantly inhibit 5-LOX-catalyzed LT, while s- hexosaminidase release was only weakly inhibited. From the extract of A. japonica, three major sesquiterpene derivatives including atractylenolide I, atractylenolide III and eudesma-4,7- dien-8-one were isolated. Among these compounds, only atractylenolide I was shown to strongly inhibit 5-LOX from RBL-1 cells. Taken together, it is concluded that A. japonica possesses the inhibitory activity on 5-LOX and atractylenolide I may contribute, at least in part, to the inhibitory activity of A. japonica.