1. The in vitro effects of centaureidin and 5,3'-dihydroxy-4'-methoxy-7-carbomethoxyflavonol (Fig. 1), two anti-inflammatory flavonoids extracted from Tanacetum microphyllum DC., have been examined on both cyclooxygenase and lipoxygenase activity. 2. These flavonoids produced an inhibition of soybean lipoxygenase activity in a dose-dependent manner, with IC50 values (20 and 29 microM respectively) similar to the reference drug. 3. The IC50 values for the in vitro inhibition of cyclooxygenase activity by these flavonoids, were higher than those that produced lipoxygenase activity (318 and 60 microM respectively). 4. These results suggest that the anti-inflammatory activity of our flavonoids may, at least in part, be due to the inhibition of leukotriene synthesis. 5. This is the first report of the biological activity in vitro of these compounds.