The effects of the extract of the root of Cynanchum wilfordi (Asclepiadaceae), the alkaloid fraction, and the isolated main constituent (gagaminine) on 5-lipoxygenase(5-LOEC 1.13.11.34) in bovine PMNL have been studied. The effect of the crude extracts of wildand cultivated plant was also compared each other. Furthermore, the inhibitory effect of gagaminine on 5-Lo was compared with those of the standard drugs. Gagaminine inhibited 5-Lo activity with an $IC_{50}$ value of $26;{mu}M$, this result indicates that gagaminine may be useful for in vivo experiments as 5-LO inhibitor.