Efforts to develop carbon nanotubes (CNTs) as a nano-vehicles for precise and controlled drug and gene delivery, as well as markers for in vivo biomedical imaging, are currently hampered by uncertainties with regard to their cellular uptake, their fate in the body, and their safety. All of these processes are likely to be affected by the purity of CNT preparation, as well as the size and concentration of CNTS use, parameters that are often poorly controlled and concentration of CNTs used, parameters that are often poorly controlled in biological experiments. It is demonstrated here in that under the experimental conditions of standard transfection methods DWNTs are taken up by cultured cells but are then released after 24 h with no discernable stress response. The results support the potential therapeutic use of CNTs in many biomedical settings, such as cancer therapy.