To identify potential harmful effects of silver nanoparticles (AgNPs) on human health, a comprehensive toxicity assay was conducted on human Jurkat T cells, using oxidative stress-related endpoint. The effect of Ag ions was also investigated and compared with that of AgNPs, as it is anticipated that Ag ions will be released from AgNPs, which may be responsible for their toxicity. Cell viability tests indicated high sensitivity of Jurkat T cells when exposed to AgNPs compared to Ag ions; however, both AgNPs and Ag ions induce similar levels of cellular reactive oxygen species during the initial exposure period and; after 24 h, they were increased on exposure to AgNPs compared to Ag ions, which suggest that oxidative stress may be an indirect cause of the observed cytotoxicity of AgNPs. AgNPs exposure activates p38 mitogen-activated protein kinase through nuclear factor-E2-related factor-2 and nuclear factor-kappaB signaling pathways, subsequently inducing DNA damage, cell cycle arrest and apoptosis. Selective toxicity of AgNPs on Jurkat T cells suggests that rigorous toxicity evaluation should be conducted using various different cell types and biological systems prior to the widespread use of AgNPs.