Antibacterial efficacies of various thermoplastics, such as medium-density polyethylene (MDPE), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) containing nano-silver colloids were studied under a wide range of testing conditions. The effects of nano-silver colloid content and the silver-polymer contact time were of our main interests and quantitatively assessed by shake flask method coupled with a plate-count-agar (PCA) technique using Escherichia coli as testing bacteria. Two different methods were used for incorporating the nano-silver colloids into the thermoplastics, these being spray-coating and melt-blending techniques. The experimental results suggested that all neat thermoplastics alone could not generally inhibit the E. coli growth, suggesting that all thermoplastics exhibited nonbactericidal behavior. However, neat PVC appeared to show a retarding effect for the E. coli growth. In addition, coating silver colloid onto all types of thermoplastic substrates could inhibit the E. coli growth up to 99.9% at the optimum silver content of 50 ppm for PS, PET and PVC and of 75 ppm for MDPE. The optimum contact time for all thermoplastics was 150 min. Among the thermoplastics used, PVC exhibited the highest % E. coli reduction, and this was confirmed by the higher silver content via Atomic Absorption Spectrometry (AAS) technique. For a given silver content, the spray-coating technique could give better dispersion level of silver throughout the thermoplastic films and this led to more effective antibacterial performance as compared with the dry-blending technique. In PVC sample, the contact angle value appeared to increase with the addition of silver content for both incorporating techniques. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.