tested negative, with or without an S9 metabolic activation. Similar negative results were reported for single-walled carbon nanotubes (SWCNT) in nonstandard Salmonella strains YG1024/YG1029 [19] and for a mixture of C60 and C70 fullerite in S. typhimurium (and E. coli) tester strains [20]. The only positive responses thus far reported were with water-soluble FePt nanoparticles capped with (CH3)4NH4OH, which induced a weak positive response in TA100 [21], and fullerene C60 dissolved in polyvinylpyrrolidone, which was photo-mutagenic in strain YG3003 [22]. The mutagenicity of AgNPs has not yet been evaluated in this mutagenicity system.