Human endothelial and lung epithelial cells were exposed to nanosized Pt shapes following acellular analyses of their oxidant potential. Despite clear evidence of particle uptake by cells, the Pt nanoparticles were not found to induce cytotoxicity or oxidative stress in either cell type. Results from in vivo respiratory tract exposures suggest that the particles are retained by lung tissue and that minimal-mild lung inflammation results from exposure to the nanosized Pt particles.