From the leaves of Epilobium angustifolium L. (Onagraceae) an antiphlogistic principle was isolated and identified as myricetin 3-O-beta-D-glucuronide. This novel flavonoid exhibited a very strong anti-inflammatory effect on carrageenan-induced edema in the rat hind paw (more than ten times more effective than indomethacin) and an inhibitory effect on prostaglandin biosynthesis (as effective as indomethacin).