The antibacterial activity of nanosilver against Gram negative Escherichia coli bacteria is investigated by immobilizing nanosilver on nanostructured silica particles and closely controlling Ag content and size. These Ag/SiO(2) nanoparticles were characterized by S/TEM, EDX spectroscopy, X-ray diffraction the exposed Ag surface area was measured qualitatively by O(2) chemisorption. Furthermore, the fraction of dissolved nanosilver was determined by measuring the released (leached) Ag(+) ion concentration in aqueous suspensions of such Ag/SiO(2) particles. The antibacterial effect of Ag(+) ions was distinguished from that of nanosilver particles by monitoring the growth of E. coli populations in the presence and absence of Ag/SiO(2) particles. The antibacterial activity of nanosilver was dominated by Ag(+) ions when fine Ag nanoparticles (less than about 10 nm in average diameter) were employed that release high concentrations of Ag(+) ions. In contrast, when relatively larger Ag nanoparticles were used, the concentration of the released Ag(+) ions was lower. Then the antibacterial activity of the released Ag(+) ions and nanosilver particles was comparable.