A new method allowing the C-14-Labeling of carboxylic acid functions of carbon nanotubes is described. The key step of the labeling process is a decarbonylation reaction that has been developed and optimized with the help of a screening method. The optimized process has been successfully applied to multiwalled carbon nanotubes (MWNTs), and the corresponding C-14-labeted nanotubes were used to investigate their in vivo behavior. Preliminary results obtained after i.v. contamination of rats revealed liver as the main target organ. Radiolabeling of NTs with a tong-Life radioactive nucleus like C-14, coupled to a highly sensitive autoradiographic method, that provides a unique detection threshold, wilt make it possible to determine for a long time period whether or not NTs remain in any organs after animal exposure.