Abstract Although several studies reported that cytotoxic effects of various nanoparticles are partially due to induction of oxidative stress, it is unclear how oxidative state of the cell per se could influence its sensitivity to cytotoxic nanoparticles. This is of clinical significance because certain pathological conditions such as inflammation is associated with elevated oxidative stress and this may alter sensitivity of cells and tissues to cytotoxic nanoparticles. Hence, this study investigated how initial exposure of BEAS-2B human bronchial epithelial cells to oxidative stress influences subsequent response to cytotoxic challenge with zinc oxide (ZnO) nanoparticles (≈10nm). Oxidative stress was induced by exposing BEAS-2B cells to 5 and 10μM of H2O2 for 45min in PBS (with Ca2+). Subsequently, the H2O2 solutions were washed off and the cells were exposed to varying concentrations (5–25μg/ml) of ZnO nanoparticles in culture media for 24h, followed by cell viability assessment with the WST-8 assay. The results demonstrated that initial transient exposure of cells to oxidative stress accentuated cytotoxicity of ZnO nanoparticles. In the negative control unexposed to H2O2, >99% of cells remained viable up to a ZnO nanoparticle concentration of 10μg/ml, but displayed a steep decrease in viability above 10μg/ml ZnO. By contrast, cells that were initially exposed to 5 and 10μM of H2O2, displayed a sharp drop in viability even at concentrations below 10μg/ml ZnO. At 10μg/ml ZnO, cells initially exposed to 10μM H2O2 displayed a viability of 40.6±2.0%, which is significantly lower than the corresponding values of 72.8±2.0% and 99.9±1.1% obtained for initial exposure to 5μM H2O2 and the negative control, respectively. Hence, initial exposure of BEAS-2B cells to oxidative stress sensitized their subsequent response to cytotoxic challenge with ZnO nanoparticles.