||Since society at large became aware of the use of nanomaterials in ever growing quantities in consumer products and their presence in the environment, critical interest in the impact of this emerging technology has grown. The main concern is whether the unknown risks of engineered nanoparticles (NPs), in particular their impact on health and environment, outweighs their established benefits for society. Therefore, a key issue in this field is to evaluate their potential toxicity. In this context we evaluated the effects on plants and microorganisms of model nanoparticles, in particular of a stable metal (Au, 10 nm mean diameter), a well-known bactericide (Ag, 2 nm mean diameter) and the broadly used Fe₃O₄ (7 nm mean diameter). The toxicity of these nanoparticles was assayed using standard toxicity tests. Specifically, germination (cucumber and lettuce), bioluminescent (Photobacterium phosphoreum) and anaerobic toxicity tests were performed. Germination tests were conducted at a NP dose of 62, 100 and 116 μg mL⁻¹ for Au, Ag, and Fe₃O₄, respectively. The bioluminscent testing (P. phosphoreum) was conducted at a dose of 28, 45 and 52 μg mL⁻¹ for Au, Ag, and Fe₃O₄, respectively. Finally, anaerobic tests were conducted at a NP dose of 10, 16 and 18 μg mL⁻¹ for Au, Ag, and Fe₃O₄, respectively. In all cases low or zero toxicity was observed. However, some perturbation of the normal functions with respect to controls in germinating tests was observed, suggesting the necessity for further research in this field. At the same time, the effect of NP-solvents was sometimes more significant than that of the NPs themselves, a point that is of special interest for future nanotoxicological studies.