Recently, cerium oxide nanoparticles (CeO2 NPs) has been reported for multi-enzyme mimetic activities like that of superoxide dismutase and catalase. Here, we report glutathione (GSH) replenishing response by CeO2 NPs in human breast (MCF-7) and fibrosarcoma (HT-1080) cells. CeO2 NPs were found to be mostly cuboidal in shape with average diameter of 25 nm. Effects on cell viability, reactive oxygen species (ROS) generation, and mitochondrial outer membrane potential (MOMP) suggested CeO2 NPs to be reasonably non-cytotoxic. Data on membrane damage and lipid peroxidation correlated well with the cell viability results suggesting NPs of CeO2 to be biocompatible. Interestingly, CeO2 NPs significantly increased intracellular GSH in cells challenged with oxidants. Replenishment of depleted GSH in oxidatively challenged cells was comparable with the GSH restoring potential of known antioxidant N-acetyl cysteine (NAC), a precursor of GSH. Like NAC, CeO2 NPs significantly replenished depleted GSH in both cell types challenged with hydrogen peroxide (H2O2) and zinc oxide (ZnO) NPs. Moreover, CeO2 NPs treated cells were significantly protected from cytotoxicity caused by H2O2 and ZnO NPs. Our findings, therefore, suggest CeO2 NPs as a potential antioxidant rather than a toxic material. (C) 2015 Elsevier Inc. All rights reserved.