Objectives: The effect of dentin on the inhibitory activity of antibacterial agents and a dental sealant was investigated.. Methods: The antibacterial effects of glutaraldehyde, sodium hypochlorite and triclosan solutions and Seal&Protect were tested against Streptococcus mutans, Streptococcus sobrinus and Lactobacillus acidophilus using the standard agar diffusion method. Additionally, bovine dentin discs ( 200 µm and 500 µm thickness) were placed between the bacteria and the test substances. After incubation at 37°C with 5% CO2, the diameters of the inhibition zones were measured. Results: The effect of 0.3% triclosan on all tester strains (100%) was reduced after passage through 500 µm dentin disc to 0% (L. acidophilus) and to 22% and 28% (S. mutans and S. sobrinus). Seal&Protect produced inhibition zones only against S. mutans, but no zone when applied on 200 µm dentin discs. The inhibition zones for 1% NaOCl and 5% glutaraldehyde against all tester strains were significantly increased when applied on dentin discs up to 230% (glutaraldehyde) and 236% (NaOCl) compared to direct application (100%). Conclusions: The inhibitory effects of the antibacterial agents tested against cariogenic bacteria were modified by dentin,either by decreasing the activity (triclosan) or by increasing it (glutaraldehyde, NaOCl). Further studies must elucidate which components of the dentin matrix are responsible for these effects.