The antimicrobial efficacy of polyamide 6/silver-nano- and microcomposites against Escherichia coli was studied as a function of the filler content. Polyamide 6 filled with 0.06 wt.% silver nanoparticles is able to eliminate the bacteria completely within 24 h. A polyamide 6/silver-microcomposite containing 1.9 wt.% of silver kills only about 80% of the bacteria during the same time. Silver ion release tests were performed to find out the reason for the much higher antimicrobial efficacy of the nanocomposite materials. The silver ion release from the polyamide 6/silver-composites obeys a zero-order rate law. The silver ion release rate increases with growing filler content. For a fixed filler content the rate of the silver ion release from the nanocomposites is about one order of magnitude higher in comparison to the microcomposites, because of the much larger specific surface area of the nanoparticles. A correlation between the silver ion release rate and the antimicrobial efficacy against E. coli was found. If the rate of the silver ion release is at least 9.5 x 10(-4) mg l(-1) cm(-2) day(-1) the silver-filled materials are able to eliminate E. coli completely within 24 h. This value is reached for polyamide 6 filled with 0.06 wt.% of silver nanoparticles. The rate of silver ion release from the microcomposites is below this threshold value even if the filler content is 1.9 wt.%. (C) 2007 Elsevier B.V. All rights reserved.