The cytotoxicity and cellular uptake of carbon nanotubes (CNTs) has recently attracted considerable interest because of the issue of biosphere-nanomaterial interactions. The biocompatibility of CNTs is determined by the metal impurities in the CNTs, the size of the CNTs and the CNT dispersion states; in particular, the type of surface modifications on the CNTs affects how they interact with cells and determines their cytotoxicity and cellular uptake. In this study, biocompatible single-walled carbon nanotubes (SWNTs) wrapped with a water-soluble copolymer, poly[2-(dimethylamino) ethyl methacrylate-co-methacrylic acid] (PDM), were prepared. We report that these SWNTs have enhanced water dispersibility and cellular internalization but no significant cytotoxic activity against mouse embryonic fibroblast NIH-3T3 cells.