Recent advances in nanotechnology led to the development of various nanostructures especially that of carbon-based nanomaterials (CBNs) for industrial and biomedical uses. As a result, the production as well as the human exposure to these materials will increase in the future. Thus, it is primordial to also know the associated health hazards. We performed in vitro cell viability studies on different types of cultured cells exposed to multiwalled carbon nanotubes and carbon nanoparticles. Our work indicates that the tested materials are toxic and that the effects of materials are cell type dependent. Light microscopic observations of the cells when exposed to the highest dose of nanomaterials showed morphological alterations, which are characteristic for cell death, and were most pronounced in the case of macrophage cells. Further experiments are aimed at elucidating the mechanisms of toxicity of CBNs. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim