The influence of dentin on the effectiveness of three antibacterial agents (triclosan, glutaraldehyde, NaOCl) on Streptococcus mutans, S. sobrinus, and Lactobacillus acidophilus was tested using the agar diffusion method with and without bovine dentin discs (200 gm and 500 mum thickness) placed between bacteria and test substances. The effect of 0.3% triclosan on all tester strains (100%) was reduced after passage through 500 Am dentin discs to 0% (L. acidophilus) and to 22% and 28% (S. mutans and S. sobrinus). Seal&Protect (Dentsply, Konstanz, Germany), a triclosan containing dental bonding agent, produced inhibition zones only against S. mutans, but no zone when applied on 200 gm dentin discs. The inhibition zones for 1% NaOCl and 5% glutaraldehyde against all tester strains were significantly increased up to 230% (glutaraldehyde) and 236% (NaOCl) when applied on dentin discs, compared to direct application (100%). Dentin may either decrease or increase the inhibitory effect of antibacterial agents.