Objective To investigate in vitro cytotoxicity and oxidative stress response induced by multiwalled carbon nanotubes (MWCNTs).Methods Cultured macrophages (murine RAW264.7 cells) and alveolar epithelium cells type Ⅱ (human A549 lung cells) were exposed to the blank control,DNA salt control,and the MWCNTs suspensions at 2.5,10,25,and 100 μg/mL for 24 h.Each treatment was evaluated by cell viability,cytotoxicity and oxidative stress.Results Overall,both cell lines had similar patterns in response to the cytotoxicity and oxidative stress of MWCNTs.DNA salt treatment showed no change compared to the blank control.In both cell lines,significant changes at the doses of 25 and 100 μg/mL treatments were found in cell viabilities,cytotoxicity,and oxidative stress indexes.The reactive oxygen species (ROS) generation was also found to be significantly higher at the dose of 10 μg/mL treatment,whereas no change was seen in most of the indexes.The ROS generation in both cell lines went up in minutes,reached the climax within an hour and faded down after several hours.Conclusion Exposure to MWCNTs resulted in a dose-dependent cytotoxicity in cultured RAW264.7 cells and A549 cells,that was closely correlated to the increased oxidative stress.