5-Methoxy-8-(2-hydroxy-3-buthoxy-3-methylbutyloxy)-psoralen (MP) is a medicinal herbal product isolated from Angelica dahurica that inhibits the cyclooxygenase-2 (COX-2)-dependent phase of prostaglandin D(2) (PGD(2)) generation in bone marrow-derived mast cells (IC(50), 23.5 microM). Western blotting with specific anti-COX-2 antibodies showed that the decrease in PGD(2) production was accompanied by a decrease in COX-2 protein levels. In addition, this compound consistently inhibited the production of leukotriene C(4) in a dose dependent manner, (IC(50), 2.5 microM). These results demonstrate that MP inhibits both cyclooxygenase-2 and 5-lipoxygenase activity. Furthermore, this compound also inhibited the degranulation reaction (IC(50), 4.1 microM). Therefore, this compound might provide a basis for novel anti-inflammatory drug development.