The uptake of multi-walled carbon nanotubes(MWNTs) by Tetrahymena pyriformis was observed with optical microscope. The viability of Tetrahymena pyriformis and its dose-effect relationship at various concentrations of the MWNTs were measured. The result revealed for the first time that MWNTs exhibited a dose-dependent growth stimulation to the cells. The rapid proliferation of cells was further confirmed by levels of the succinate dehydrogenase (SDH)

protein and nucleic acid content per 104 cells of control and treated groups. TGA analysis showed the nonspecific binding of proteose peptone to the MWNTs. Thus we proposed that a great deal of proteose peptone

carried by MWNTs inside the cells

might be a result of the rapid proliferation of Tetrahymena pyriformis.