Carbon nanotubes currently attract intense attention for various biomedical applications due to their large surface areas, high electrical conductivity, and excellent strength. However, these attractive properties of nanomaterials are also the main factors for their potential toxicity. The present study was undertaken to determine the toxicity exhibited by multi- walled carbon nanotubes (MWCNT) in A549 lung epithelial cells. Treatment with increasing doses of MWCNT decreased the cell viability. The glutathione concentration decreased and intracellular reactive oxygen species (ROS) increased in a dose- and time- dependent manner, suggesting that the cytotoxicity on A549 cells was due to oxidative stress. Expression of heme oxygenase (HO)-l, a redox regulator and heat shock protein, increased with dose after MWCNT treatment. Pretreated with zinc protoporphyrin IX (ZnPP IX), a competitive HO inhibitor, underwent cell viability decrease and ROS accumulation increase to a greater extent than cells with MWCNT treatment alone, but these effects were reversed by co-treatment with bilirubin, a product of HO catalysis. Taken together, these findings suggest that MWCNT induce oxidative stress and HO-1 expression in A549 cells, and HO-1 induction may confer a cellular adaptive response against MWCNT-induced cytotoxicity.