The pharmacological effects of a new anti-inflammatory compound, alpha-(3,5-di-tert-butyl-4-hydroxybenzylidene)-gamma-butyrolactone (KME-4), and its inhibitory effects on arachidonate prostaglandin synthetase and 5-lipoxygenase activities were examined. KME-4 showed anti-inflammatory activity. It was less active than indomethacin, but more active than naproxen and ibuprofen in carrageenin-induced paw edema in rats; and it was less active than indomethacin, equipotent as naproxen, but more active than ibuprofen in granuloma formation in rats. The ulcerogenic activity of KME-4 was weaker than indomethacin and naproxen, but stronger than ibuprofen in starved rats. The ratio of UD50 stomach to ED30 carrageenin edema or to ED25 granuloma for KME-4 showed higher values than those of the reference drugs. KME-4 showed antipyretic activity in yeast-induced fever in rats. It also inhibited platelet aggregation induced by arachidonic acid and protected rabbits from arachidonic acid-induced death. Furthermore, KME-4 was found to be equipotent in inhibiting both prostaglandin synthetase and 5-lipoxygenase activities of rat basophilic leukemia cells, unlike indomethacin, naproxen and ibuprofen. It also inhibited the prostaglandin synthetase activity of bovine seminal vesicle. The present findings indicate that KME-4 may be a new type of anti-inflammatory drug with dual prostaglandin synthetase and 5-lipoxygenase inhibition.