A biological membrane is the front line of defense for cells against various environmental stresses such as heat and reactive oxygen species (ROS) and is expected to play an important role in the antioxidant system with antioxidant enzymes, similarly to its chaperone-like function in cooperation with heat shock proteins. The oxidative stress response of superoxide dismutase (SOD), which is known to catalyze the dismutation of O-2(-) to H2O2 was investigated in the presence of artificial membranes, liposomes, in order to obtain fundamental information on the biological ROS scavenging system. SOD lost its activity in the presence of H2O, and was found to have two loops including one which contains an a-helix which presents the substrate O-2(-) to the activity center of SOD (Cu(II)). From circular dichroism analysis of SOD in the presence of H2O2, the contents of the a-helix in SOD were found to decrease in correspondence with the inactivation and conformational change of SOD, suggesting that the conformation of the a-helix loops affects SOD activity. In the presence of liposomes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), SOD was not inactivated in the presence of H2O2 although the contents of its a-helix structure were decreased. The oxidized SOD was found to interact with the liposome surface under oxidative stress using dielectric dispersion analysis. Based on these results, a possible mechanism of SOD protection against ROS on liposomes was presented.