Excess production of reactive oxygen species by macrophages has been implicated in many inflammatory diseases. The present study investigated the inhibitory effect of the stem bark extract of Acanthopanax senticosus on the production of superoxide anion and hydrogen peroxide in mouse peritoneal macrophages in vitro and in vivo. Exposure of mouse peritoneal macrophages to A. senticosus extract significantly suppressed superoxide anion production induced by zymosan in a dose-dependent manner. Similarly, exposure of mouse peritoneal macrophages to A. senticosus extract significantly inhibited hydrogen peroxide production induced by phorbol 12-myristate 13-acetate (PMA) in a dose-dependent manner. Intraperitoneal administration of A. senticosus extract to KM mice reduced the ex vivo production of zymosan induced-superoxide anion and PMA-induced hydrogen peroxide by their peritoneal macrophages. Exposure to A. senticosus extract did not affect the cell viability or systemic toxicity. A. senticosus inhibited reactive oxygen species production by mouse peritoneal macrophages in vitro and in vivo and may be partly responsible for the antiinflammatory function.