Ag nanoparticles were added (39-5000 mu g/L) to cultures of Desmodesmus subspicatus and Pseudokirchneriella subcapitata to obs. the toxic effects under conditions of the OECD 201, growth inhibition test. The growth of the Chlorophyceae was partially reduced at concns. of Ag nanoparticles about 30 mu g/L and nearly completely inhibited at concns. about 220 mu g/L.