Several naturally occurring prenylflavones were tested for their inhibitory actions on arachidonate 5-lipoxygenase purified from porcine leukocytes. Of the compounds tested, artonin E (5'-hydroxymorusin) exhibited the most potent inhibition on arachidonate 5-lipoxygenase (IC50 = 0.36 microM). Arachidonate 12-lipoxygenase purified from porcine leukocytes and human platelets, 15-lipoxygenase from rabbit reticulocytes and fatty acid cyclooxygenase from bovine vesicular glands were inhibited by the compound only at higher concentrations (IC50 = 2.3, 11, 5.2 and 2.5 microM, respectively).