The cytotoxicity of different samples of carbon nanotubes synthesised by catalytic chemical vapour deposition was investigated towards human umbilical vein endothelial cells, using two cytotoxicity standard tests (neutral red assay for the cell viability and MTT assay-tetrazolinium salt-for the cell metabolic activity). No toxicity was found for any sample, although a slight dilution effect may exist for two of them. (c) 2006 Springer Science + Business Media, Inc.