Due to their unique physical properties, carbon nanotubes are becoming promising novel materials in diverse areas like information technology, ultrastiff materials, biomedicine etc. The toxicological study of these materials is very imperative for the safety assessment in respect to their wide applications. The objective of the toxicity study was to find out whether water soluble multi-walled carbon nanotubes (S-MWNTs) would impair the phagocytic activity of macrophages, which play an important role in defenses against invading microbioles. The results showed that S-MWNTs did not impair the phagocytosis of macrophages; to our surprise, S-MWNT significantly enhanced this function. Increased activity of enzymes in lysosome also showed that S-MWNTs enhanced the lysosome function of macrophages. However, S-MWNTs can not influence nitric oxide secretion in macrophages and induce inflammation.