The need for toxicological studies on carbon nanotubes (CNTs) has arisen from the rapidly emerging applications of CNTs well beyond material science and engineering. In order to provide a method to collect data about toxicology, we characterized by Scanning Electron Microscopy (SEM), by Energy Dispersive X-ray Spectrometry (EDS) analysis and by Focused Ion Beam (FIB) microscopy different kinds of treated CNTs. The bio-interaction was investigated seeding Crandell feline kidney fibroblasts with CNT-modified medium; a dedicated sample preparation by FIB has been defined to fix cells. In the present study, the cytotoxic effects of CNTs with 91% and 97% of purity were compared and changes in the growth behaviour of cells after 3 days in culture with modified medium have been recorded, considering also the distribution of CNTs within cells. While lower purified CNTs induced a slight cytotoxic effect, homogeneously suspended CNTs with high purity were less cytotoxic, and the rate of cell growth remained constant. CNTs aggregated in bundles, showed high adhesion on cell membrane. Interestingly, CNTs bundles were observed inside cells, underneath the cell membrane, and despite of that, cells were extended, in good vitality conditions and no cell-degeneration was observed.