Carbon nanotubes are considered to be one of the novel most attractive materials in nanotechnology. Because of their multiple industrial and biomedical applications, thorough studies on their toxicity and biocompatibility become a priority in order to prevent possible health risks. In this study the effects of multiwalled carbon nanotubes (MWCNT) on healthy monocytes from human peripheral blood were investigated. The results indicate that MWCNT exert a cytotoxic effect on monocytes, inducing cell death and increasing the extent of apoptosis induced by a chemotherapic agent. This cytotoxic effect may have important implications, and much attention in terms of evaluation of exposure risks is recommended.