The development of carbon nanotubes (CNTs) for biomedical and biotechnological applications has gained great promise recently, especially for their interesting use in the delivery of therapeutically active molecules to targeted cells. The interaction between cells and this nanomaterial is a critical feature that is responsible for the pharmacological effect as well as for any eventual toxicity. With respect to the latter aspect, in this manuscript we have evaluated a few parameters that seem to be involved in the cytotoxic profile of CNTs (both Single-walled (SWCNTs) and Multi-walled (MWCNTs) nanotubes), such as their sidewall functionalization, tubes' length, solubility, concentration and purity. Among them, we identified the last as the most crucial factor: we have shown that our ultrapure, totally dispersible, carbon nanotubes not only display lack of toxicity in the range of concentrations normally used (10-150 μg/ml), but they also pave the way for an extensive use of this material for several biomedical purposes.