Carbon nanotubes have various features, and many industries have an interest in carbon nanotubes. Recently, however, artificial nanoparticles have become a public health concern. Current experiments have indicated that the survival rate of cultured cells gradually declined by exposure to single-walled carbon nanotubes (SWCNTs) despite SWCNT purification. These results suggest that SWCNTs have not only the toxicity of catalyst metals but also their own toxicity. However, SWCNTs did not induce DNA mutagenesis, which means that SWCNTs do not transform cells directly from a normal to malignant state. In contrast, the uptake process of SWCNTs was clarified with a transmission electron microscope (TEM). SWCNTs contained other risk of toxicity because of the needlelike structure and persistence in biological systems. TEM observation revealed that SWCNTs brought into cells by clathrin-mediated endocytosis were transported subsequently to the lysosome. Such accumulated SWCNTs appeared to injure the cells in several ways that pierced the organelle and broke the membrane during cytokinesis or movement. SWCNTs that pierced the cell membrane were found frequently.