This article is concerned with the effects of nanosized silver colloids on the antibacterial properties of silk fibers against two kinds of bacteria: Staphylococcus aureus and Escherichia coli. Different concentrations of silver nanoparticles (Ag NPs; 10, 25, 50, and 100 ppm) were applied to silk fibers by an exhaust method. The effect of medium pH on the Ag NP uptake on the fibers was studied. Also, sodium carbonate and sodium chloride were added to the liquor as auxiliaries. Scanning electron microscopy was used to observe the morphology of the silk fibers. The antibacterial activity was examined by a bacterial counting method. Energy-dispersive X-ray spectroscopy was also used to show the elements on the surface of the silk fibers. We observed that the antibacterial activity increased with silver treatment. It also increased with decreasing pH, especially for the raw silk. The use of NaCl improved the uniformity of the Ag NPs on the fiber surface and increased the antibacterial activities. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 253-258, 2010