Effect of 24 h treatment with 50, 80 and 200 nm Ag ENMs (0.21 -15.6 μg/ cm2) on induction of hprt gene mutations in V79- 4 cells ( bar graphs, left hand scale). The mutant frequencies (x106) are expressed as the mean ± SD of two independent experiments, with two independent harvests per experiment. Horizontal line shows hprt gene mutant frequency in untreated cells (8.47 ± 3.83). MMS (0.03 μM, 30 min), a positive control gave 80.03 ± 22.13 hprt gene mutations. Significant difference from unexposed control (*p < 0.05, **p < 0.01, ***p < 0,001). Cytotoxic effects – Plating efficiency (PE) (horizontal marker line, right hand scale) is expressed as the mean of two independent experiments. Results represent cytotoxicity relative to 100% of the negative control. Cytotoxicity of MMS has not been observed (PE = 97.99 ± 6.14%).