To test the impact of commercial nanomaterials on the environment and biological species, the potential eco-toxicity of nano sized silver powder was investigated with bacteria, Gram-negative Escherichia coli and Gram-positive. Bacillus subtilis, and yeast, Saccharomyces cerevisiae. When commercial silver nanopowder was dispersed in water, it was shown to contain both silver nanoparticles and silver ions. S. cerevisiae showed a higher survival rate than the other two species. To compare the antimicrobial activity as a quantitative parameter, the susceptibility constant was determined. The susceptibility constant of the silver ions were larger than those of the silver nanopowder. On average, the degree of susceptibility to silver decreased in the following order, E. coli > B. subtilis > S. cerevisiae.