<jats:title>Abstract</jats:title><jats:p>The chloroform and the ethyl acetate fractions from the roots of <jats:italic>Acanthopanax chiisanensis</jats:italic> exhibited the significant inhibition of TPA‐induced prostaglandin E<jats:sub>2</jats:sub> (PGE<jats:sub>2</jats:sub>) production in rat peritoneal macrophages. Five lignans were isolated from the chloroform fraction and their structures were elucidated as <jats:italic>l</jats:italic>‐sesamin, helioxanthin, savinin, taiwanin C, and 3‐(3,4‐dimethoxybenzyl)‐2‐(3,4‐methylenedioxybenzyl)butyrolactone. Among the lignans tested, taiwanin C showed the most potent inhibitory activity (IC<jats:sub>50</jats:sub> = 0.12 µM) on PGE<jats:sub>2</jats:sub> production with the relative order of potency, taiwanin C ≫ 3‐(3,4‐dimethoxybenzyl)‐2‐(3,4‐methylenedioxybenzyl)butyrolactone > savinin = helioxanthin. <jats:italic>l</jats:italic>‐Sesamin showed no inhibitory activity at 30 µM. Copyright © 2005 John Wiley & Sons, Ltd.