Inhibition of 5637 cell survival after 24-hour exposure to various concentrations of gold nanoparticles (AuNPs) (0, 1.56, 3.12, 6.25, 12.5, 25, and 50 µg/ml) according to the results of MTT assay. Asterisks indicate significant difference compared with the control cells (*p<0.05, **p<0.01, ***p<0.001). The results are shown as mean ± SD of three individual 96-well plates (n=3).