The aerial parts of Tanacetum microphyllum D.C. (Compositae), used in folk medicine as an anti-inflammatory and anti-ulcer agent, were studied, the active principle hydroxyachillin (1) was isolated, and its pharmacological properties were analyzed. Hydroxyachillin showed marked anti-inflammatory activity in carrageenan-induced paw edema in mice. In the experimental models in vitro, hydroxyachillin has been reported to be a dual inhibitor of cyclooxygenase and soybean lipoxygenase. In this study, inhibition of prostaglandin synthetase (bovine seminal vesicular microsomes) by hydroxyachillin required concentrations more than twice those which inhibited soybean lipoxygenase (ED50 values = 10(-4) M and 2.6 x 10(-5) M, respectively). Therefore, an inhibition of leukotriene synthesis might be involved in the anti-inflammatory activity of hydroxyachillin in vivo.