The topical anti-inflammatory activity of Thymus willdenowii Boiss (Labiatae) leaves, a herbal drug used in Moroccan folk medicine, has been studied using the croton oil ear test in mice. A bioassay-oriented fractionation procedure showed that the activity concentrates in the chloroform extract, which has a potency similar to that of indometacin, the non-steroidal anti-inflammatory drug used as reference (ID50 (dose giving 50% oedema inhibition) = 83 microg cm(-2) and 93 microg cm(-2), respectively). The main compounds responsible for the anti-inflammatory activity of T. willdenowii are ursolic acid and oleanolic acid. The flavonoids luteolin-3'-O-glucuronide and eriodictyol-7-O-glucoside were found for the first time in the genus Thymus.