Advances in nanotechnology in various fields of human activity contribute to increase of their production, improved properties and wider implementation of nanomaterials. However, increasing use may enhance their release into the environment and can lead to affecting human health. The toxicity of colloidal solutions of metal nanoparticles (Cu, Mn) and their oxides, obtained in the absence and presence of a stabilizer, was examined and compared with the use of the standard test system of Allium cepa L.. The phytotoxicity of the experimental solutions was evaluated according to the growth response of the onion roots; the cyto- and genotoxicity were estimated due to the proliferative activity of the root meristem cells. It was established that solutions of stabilized metal nanoparticles were at given concentration toxic to Allium cepa L. according to the integral index of roots growth, however, were not cytotoxic. Difference in the phytotoxicity of stabilized and non-stabilized metal nanoparticles and their oxides depended on their phase composition and affected root growth.