Amorphous silica is increasingly used in diagnostic and biomedical research because of its ease of production and relatively low cost. It is generally regarded as safe and has been approved for use as a food or animal feed ingredient. Recent literature reveals that amorphous silica may present toxicity concerns at high doses. In anticipation of potential human exposure to silica, it is advisable to examine its toxicity to cells of different organs. Consequently, we investigated the response of several normal fibroblast and tumor cells to varying doses of amorphous silica or composite nanoparticles of silica and chitosan. A cell proliferation assay indicates that silica nanoparticles are nontoxic at low dosages but that cell viability decreases at high dosages. A lactate dehydrogenase (LDH) assay indicates that high dosages of silica induce cell membrane damage. Both assays reveal that fibroblast cells with long doubling times are more susceptible to injury induced by silica exposure than tumor cells with short doubling times. In contrast, silica-chitosan composite nanoparticles induce less inhibition in cell proliferation and less membrane damage. This study suggests that the cytotoxicity of silica to human cells depends strongly on their metabolic activities but that it could be significantly reduced by synthesizing silica with chitosan.