Glutathione peroxidase might play an important role in the protection of cellular structures against oxidative challange by hydrogen peroxide and several organic hydroperoxides. It is widely accepted that allicin is biological active component of garlic, and allicin is easily degraded to diallyl disulfide and other components. This study was attempted to elucidate the effect of diallyl disulfide on some biological activities. It was observed that the activity of serum transaminase and glutathione level in liver were not changed by the treatment of diallyl disulfide. The liver cytosolic glutathione peroxidase activity was significantly enhanced. Whereas, mitochondrial enzyme activity was slightly increased. In the presence of diallyl disulfide in vitro, Vmax value of glutathione peroxidase for hydrogen peroxide was increased. On the other hand, Km value was not changed.