Extensive production and wide application of TiO₂ nanoparticles has stimulated research on its potential biological effects on different groups of organisms but the interaction of TiO₂ nanoparticles with higher plants remains poorly understood. We have studied the effect of TiO₂ nanoparticles on Allium cepa using a modification of the conventional Allium test with nanoparticles suspended in distilled water as opposed to growth medium. Nanoparticulate TiO₂ was found to have low toxic potential and the mitotic index was among the most sensitive measures of the effect of nano-TiO₂. We conclude that modified Allium test is suitable to provide comparative data on the biological potential of a variety of nanoparticles and could be used in a tiered approach to nanotoxicity testing.