The development of nanomaterials for biomedical and biotechnological applications is an area of research that holds great promise and intense interest(1), and carbon-based nanostructures in particular, such as carbon nanotubes (CNTs), are attracting an increasing level of attention(2,3). One of the key advantages that CNTs offer is the possibility of effectively crossing biological barriers, which would allow their use in the delivery of therapeutically active molecules. Our laboratories have been investigating the use of CNTs in biomedical applications, and in particular as nanovectors for therapeutic agent delivery(4-8). The interaction between cells and CNTs is a critical issue that will determine any future biological application of such structures. Here we show that various types of functionalized carbon nanotubes (f-CNTs) exhibit a capacity to be taken up by a wide range of cells and can intracellularly traffic through different cellular barriers.