OBJECTIVE Bacteria persisting in the root canal system may thwart endodontic therapy. It is therefore interesting to know whether clinically available root canal sealers have an antimicrobial effect. The objective of the present in vitro study was to investigate the antibacterial effect of various sealers on the endodontologically detectable species Enterococcus faecalis (E. faecalis), Fusobacterium nucleatum (F. nucleatum) and Porphyromonas gingivalis (P. gingivalis). METHODS The antibacterial effectiveness of the sealers was tested by means of the agar diffusion test (ADT) and the direct contact test (DCT). Eight different sealers (AH Plus(®), Hermetic(®), RoekoSeal(®), Sealapex(®), Apexit Plus(®), 2Seal(®), EndoREZ(®) and ProRoot MTA(®)) and two temporary sealers (Calxyl(®) and Gangraena Merz(®)) were tested. At first, 100μl of bacterial suspension (BS) of each individual micro-organism (optical density (OD) 0.5) was applied separately to Schaedler agar plates for the ADT. Subsequently, freshly mixed and set sealer was applied. After 48h of incubation, the inhibition zones were measured. Further, 18mg of sealer were put into each well of 48-well cell culture plates and overlaid with 400μl of Schaedler liquid medium and 100μl of BS (OD 0.5) in monoculture. Bacterial growth was determined by the DCT from the optical density of the liquid by photospectrometry after 2, 4, 6, 8, 10, 12 and 24h. RESULTS For the application, the sealer Hermetic(®), a significant suppression of the species E. faecalis, F. nucleatum and P. gingivalis was detected in both the ADT and the DCT. AH Plus(®) showed a suppressive effect on E. faecalis and F. nucleatum in the DCT. With all other sealers tested, E. faecalis was not suppressible. RoekoSeal(®), Calxyl(®) and Gangraena-Merz(®) showed no antibacterial effect on the tested species whatsoever. CONCLUSIONS We have shown in both ADT and DCT that some root canal sealers suppress the growth of E. faecalis in vitro.