Carbon nanotubes (CNTs) are attracting significant attention as a novel material for future innovations. Many in vitro studies have assessed the cytotoxicity of CNTs, but the effects of CNTs differ depending on the cell lines and the synthetic method adopted for fabricating CNTs. In the present study, the differential effects of single-walled CNTs (SWCNTs) on the cell viability of A549 cells from human lung carcinomas and FaDu cells from human head and neck carcinomas were investigated. The SWCNTs used in the present study were synthesized with nickel and yttrium (SO-SWCNTs), and iron (FH-P-SWCNTs) as catalysts. Cell viability was evaluated on the basis of cell-membrane biomass, adenosine triphosphate (ATP) content, and intracellular metabolic capacity. After 24-hr exposure of A549 and FaDu cells to 1.0 mg/ml SO-SWCNTs, the cell-membrane biomass of A549 cells decreased to 43% as compared to the control cells, whereas that of FaDu cells remained over 90%. After 24-hr exposure of A549 and FaDu cells to 1.0 mg/ml SO-SWCNT, the intracellular metabolic capacity decreased to 24% and 37%, respectively, and the ATP content decreased to 40% and 54%, respectively. SWCNTs had a greater impact on the viability values of A549 cells than on those of FaDu cells. In addition, cells exposed to FH-P-SWCNTs exhibited a higher viability than those exposed to SO-SWCNTs. Caspase 3/7 activity was not increased at maximum concentration of 1.0 mg/ml SO-SWCNTs. It was surmised that sensitivity to SWCNTs differs among the 2 cell lines; additionally, SWCNT characteristics may produce different effects on these cell lines.