This study was intended to clarify the protective mechanism of diallyl disulfide on the carbon tetrachloride-induced hepatotoxicity in mice. It was observed that a powerfully increment of serum alanine aminotransferase activity and hepatic lipid peroxide content after carbon tetrachloride injection were markedly inhibited by the pretreatment of diallyl disulfide (20mg/kg) for 5 days. It was also observed that hepatic aminopyrine demethylase and xanthine ocidase as free radical generating enzymes as well as superoxide dismutase and catalase activities as free frdical scavenging enzymes and hepatic glutathione content were not changed by the pretreatment with diallyl disulfide. But, treatment with diallyl disulfide did signifiantly increase cytosolic glutathione S-transferase activity. However, glutathione S-transferase activity in the presence of diallyl disulfide was not affected in vitro. Therefore, it is concluded that mechanism for the observed preventive effect ofdiallyl disulfide against the carbon tetrachloride-induced hepatotoxicity can be due to the engancement of glutathione S-transferase activity.