The toxicity of silica (SiO2) and PVP-coated silver (Ag) nanoparticles (NPs) was investigated in two pairs of human or mouse cell lines originating from lung epithelium (A549 and ASB-XIV) and macrophages (THP-1 and J744A.1). Both NPs were characterized in H2O and cell media and demonstrated to be well dispersed. The primary sizes were 69 nm (Ag) and 27 nm (SiO2) as determined by TEM. Cytotoxicity was tested after 24 h in terms of viability by dehydrogenase activity (WST-8), apoptosis (Annexin V/PI) and the formation of ROS (DCF). Murine cells are more sensitive to NPs than human cells of similar origin, and the toxic response depends on both the NP type and the cell type. Approximately 10- times higher doses were needed for SiO2 NPs to cause a 50 % decrease in cell viability compared to Ag NPs. Significant increases in ROS generally occured at doses close to EC50 or higher leaving the question whether increased ROS were caused by the NPs or as a consequence of cell death. Induction of ROS was also assessed by the comet assay and modifications of DNA. In both human and murine epithelial lung cells, the EC50 NP concentrations from the WST-8 assay correlated well with results from the annexin V/PI assay. Death at EC50 in the lung cells was equally due to apoptosis and necrosis after exposure to either NP. However, large discrepancies were found when comparing EC50 values from the WST-8 assay in macrophages to results from the Annexin V/PI assay. The WST-8 assay appeared to overestimate cell death caused by Ag NPs in J774A.1 cells and by SiO2 NPs in THP-1 cells, whereas the assay underestimated the EC50 values of SiO2 NPs in J774A.1 cells and Ag NPs in THP-1 cells. These discrepancies suggest that dose-responses based on tetrazolium dyes such as WST-8 should be confirmed by additional assays e.g. annexin V/PI. Our preliminary data suggest that NP mediated toxicity can be higher in murine cell lines compared to their human counterparts. This information could be of importance if risk assessment will be based upon animal experimentation. This association will be the subject of investigation in our future work.