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 ($IC_{50}$, $23.5;{mu}M$). 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, ($IC_{50}$, $2.5;{mu}M$). These results demonstrate that MP inhibits both cyclooxygenase-2 and 5- lipoxygenase activity. Furthermore, this compound also inhibited the degranulation reaction $(IC_{50}$, $4.1;{mu}M$). Therefore, this compound might provide a basis for novel anti-inflammatory drug development.