Cytotoxicity induced by silver nanoparticles (AgNPs) and the role that oxidative stress plays in this process were demonstrated in human hepatoma cells. Toxicity induced by silver (Ag+) ions was studied in parallel using AgNO3 as the Ag+ ion source. Using cation exchange treatment, we confirmed that the AgNP solution contained a negligible amount of free Ag+ ions. Metal-responsive metallothionein 1b (MT1b) mRNA expression was not induced in AgNP-treated cells, while it was induced in AgNO3-treated cells. These results indicate that AgNP-treated cells have limited exposure to Ag+ ions, despite the potential release of Ag+ ions from AgNPs in cell culture. AgNPs agglomerated in the cytoplasm and nuclei of treated cells, and induced intracellular oxidative stress. AgNPs exhibited cytotoxicity with a potency comparable to that of Ag+ ions in in vitro cytotoxicity assays. However, the toxicity of AgNPs was prevented by use of the antioxidant N-acetylcysteine, and AgNP-induced DNA damage was also prevented by N-acetylcysteine. AgNO3treatment induced oxidative stress-related glutathione peroxidase 1 (GPx1) and catalase expression to a greater extent than AgNP exposure, but treatment with AgNO3 and AgNPs induced comparable superoxide dismutase 1 (SOD1) expression levels. Our findings suggest that AgNP cytotoxicity is primarily the result of oxidative stress and is independent of the toxicity of Ag+ ions.