Carbon nanotubes (CNTs) are widely used for biomedical applications as intracellular transporters of (bio)molecules, due to their high propensity to cross cell membranes. However, there is a clear discrepancy in the literature about their uptake mechanism, which should be related to the differences existing in the nanotube materials, as well as the experimental procedures. Despite the fact that there are some studies on the influence of the CNT surface chemistry, the role of the properties of non-functionalized CNTs in cellular uptake has not been much investigated to a great extent. In this work, different kinds of multi-wall CNTs (MWCNTs) are produced and fully characterized, in terms of diameter, length, metal impurity, carbon soot and surface chemistry. These MWCNT samples are tested in vitro, and the cellular uptake is indirectly evaluated by using standard fluorescent probes and confirmed by TEM images. Our assays demonstrate that nanotube length clearly influences their uptake and shorter (sub-1 mu m) MWCNTs are easier to be internalized through an energy-independent pathway. The results of this investigation may be useful for the design of promising CNT-based vectors for cell therapy. (C) 2008 Elsevier Ltd. All rights reserved.