Aim: Bacterial growth into obturated root canals may occur when coronal restorations are lost or their seal compromised. The aim of this study was to test the hypothesis that incorporation of quaternary ammonium polyethyleneimine (PEI) nanoparticles into endodontic sealers will render them antibacterial properties so that they may prevent such bacterial growth. Materials and methods: PEI nanoparticles were incorporated at 2% w/w into AH 26 and AH Plus or Roth's and Pulp Canal Sealer (PCS) sealers. Antibacterial activity of the sealer surface against Enterococcus faecalis was measured immediately after setting and after ageing for 4 weeks, using the Direct Contact Test. Results: All four PEI-containing sealers presented high early antibacterial properties that were fully maintained after ageing for 4 weeks. None of the original unmodified sealers presented such activity after ageing (P 0.001). Conclusions: PEI nanoparticles may be used to induce stable antibacterial properties in the surface of both epoxy-based and zinc-oxide-eugenol-based endodontic sealers. Keywords: antibacterial, DCT, endodontic-sealer, E. faecalis, nanoparticles, PEI