Because glutathione scavenges reactive oxygen species (ROS) and also donates electrons to antioxidative systems, it may compensate for the oxidative stress caused by SOD1 deficiency. The cystine/glutamate transporter, which consists of two proteins, xCT and 4F2hc, has been designated system xc−. This transporter system plays a role in the maintenance of glutathione levels in mammalian cells. In the present study, we created SOD1−/−; xCT−/− double-knockout mice by intercrossing xCT-knockout and SOD1-knockout animals. We determined if the double-knockout mice express the phenotypic characteristics unique to SOD1−/− mice—increased oxidative stress and the production of autoantibodies against erythrocytes. We also compared the phenotype of the double-knockout mice with those of the single-knockout and wild-type mice. Although two major antioxidative systems were found to be defective in the SOD1−/−; xCT−/− mice, relative to the SOD1−/− mice, no functional deficits were observed. Based on these results, it appears that defects in system xc− do not exacerbate the phenotypic consequences of SOD1 deficiency in postnatal mice under ordinary breeding conditions.