The antimicrobial activity of metallic nanoparticles against bacteria was investigated. Silver, copper, and titania nanoparticles were tested. The bacteria species E. coli and B. subtilis were selected as models for Gram-negative and Gram-positive bacteria, respectively. Bacteriological tests were performed in Luria-Bertani (LB) medium or Nutrient (NT) medium on solid agar plates containing different concentrations of nanoparticles in the range of 0 to 150 μg/mL. Silver and copper nanoparticles were shown to be an effective bactericide. However, titania nanoparticles were found to be ineffective as an antimicrobial agent. It appears that E. coli cells are more sensitive to the copper nanoparticles than to silver nanoparticles and B. subtilis cells have similar sensitivity against both copper and silver nanoparticles. Keywords : Antimicrobial, Bacteria, B. subtilis, E. coli, Nanoparticles, Silver, Copper