Phytotoxicity of colloidal solutions of metal-containing nanoparticles (Ag, Cu, Fe, Zn, Mn) was investigated using a standard Allium cepa (L.) test system. Toxicity of experimental solutions at the organism level was evaluated in terms of biomass growth of onion roots, and cytotoxicity was estimated by proliferative activity of root meristem cells. The colloidal solutions of metal nanoparticles inhibited the growth of Allium cepa (L.) roots due to their ability to penetrate into cells and interact with their components and, thus, to inhibit mitosis. According to our results, cytotoxicity of test solutions decreases in the following order: Cu ≥ Zn ≥ Ag ≥ Fe. Solution of Mn-containing nanoparticles contributed to root growth reaction.