As a new class of xenogenous nanoparticle,quantum dots(QDs) possess the potential to co-exist with Cu2+ in human liver.The combined toxicity is thus concemed.Considering QDs and Cu2+ are known ROS(reactive oxygen species) inducer,we investigated the combined oxidative stress and corresponding protective strategy using human hepatic L02 cells.The results demonstrated that the presence of a small amount of MPA-CdTe QDs(2μg/Ml) in a Cu2+ solution(2.5-20μg/Ml) resulted in a higher toxicity with up to 8-fold cell viability decrease,which was accompanied by cell morphology changes.The combined toxicity was then confirmed as ROS associated oxidative stress with up to 300% and 35% increase of the intracellular ROS level and glutathione S-transferase(GST) activitv.respectively.N-acetylcysteine(NAC) can also provide almost complete protection against the induced toxicity.Therefore,the ROS associated oxidant injury might be responsible for the QDs-Cu2+/Cu2+ induced toxicity and could be balanced through cytoprotective antioxidnt enzyme GST.