Triclosan was used as antibacterial agent in various vinyl thermoplastics and calcium carbonate (CaCO3)/thermoplastic composites and the antibacterial performances were studied through Halo and Plate-Count-Agar (PCA) test methods. The thermoplastics used were polyethylene (LDPE, MDPE, HDPE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). Escherichia coli (E. coli, ATCC 25922) and Stapphylococcus aureus (S. aureus, ATCC 25923) were used as the testing bacteria. The color index results suggested that introducing triclosan did not change the color of all thermoplastics used. The antibacterial results showed that the inhibition zone increased with increasing triclosan for nonpolar thermoplastics like LDPE, MDPE, HDPE, PP, and PS films whereas the opposite effect was observed for polar PVC film. The antibacterial efficacies of the triclosan decreased in the order of LDPE > MDPE > HDPE > PP > PS > PVC and this was confirmed by the triclosan releasing and FT-IR results. The differences in the antibacterial performances of the studied thermoplastics with triclosan were associated with their rigidities, abilities to crystallize, and free volume or molecular density. The sensitivities of E. coli and S. aureus bacteria to the triclosan were found to be dependent on the testing methods used for the antibacterial performance evaluations. The addition of CaCO3 worsened the antibacterial performances in the triclosan filled HDPE and PS blends, but had a benefit for improved bacterial reduction in the triclosan-filled PVC blend. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 253-261, 2011