Background. The authors conducted an in vitro study to determine the antimicrobial and antifungal activity of a recently introduced thermoplastic, synthetic, polymer-based polyester root canal core material (Resilon, Resilon Research, Madison, Conn.) against five different microorganisms by means of the agar diffusion test over different periods. Methods. The microorganisms tested were Enterococcus faecalis, Pseudomonas aeruginosa, Staphylococcus aureus, Porphyromonas endodontalis and Candida albicans. A microbiologist transferred Resilon cones and gutta-percha cones to the inoculated agar and incubated them at 37 C, either aerobically or anaerobically, as required for optimal growth. Results. The Resilon cones exhibited no antimicrobial effect against any of the bacteria tested, except for S. aureus. It showed antimicrobial efficacy against S. aureus during the first 24-hour period (P < .05). However, after 48 and 72 hours, Resilon cones no longer inhibited the growth of S. aureus. In addition, the material demonstrated no antifungal activity during any of the three testing periods. Conclusion. The results of this study indicate that the antibacterial and antifungal efficacy of the Resilon cone is not superior to that of conventional gutta-percha. Clinical Implications. Clinicians should not use the new root canal core material for its antimicrobial or antifungal efficacy.