The in vitro cytotoxicity of both the multiwalled carbon nanotubes (MWCNT) in suspension with culture medium and the tetracalcium phosphate/monetite cement with addition of 0.8 wt% of MWCNTs on fibroblasts and osteoblasts were studied. The cytotoxicity was evaluated by MTS test (formazan) and live/dead staining. No cytotoxicity of MWCNT extract was measured contrary to about 60% reduction in proliferation of fibroblasts in MWCNT suspension as compared with negative control. The several contact cytotoxicity of MWCNT composite cement surfaces on seeded cells was demonstrated by MTS test and live/dead staining of damaged fibroblasts and dead osteoblasts after 72 h of culture. The detailed microstructure analysis showed a significant refinement of the surface texture due to the formation of thin needle-like hydroxyapatite particles on MWCNTs and this effect could be responsible for cytotoxicity of composites.