The diffusion gradient which develops across dentine and the clearance into the capillary circulation at the pulpal side of the tissue both reduce the concentration of potential toxins applied to dentine, thus protecting pulpal cells. The data presented on eugenol indicates that when release of a potential toxin from a solid material is hydrolytic the limited wetness of dentine, i.e. the limited availability of water for hydrolysis, contributes to the protective effect of the tissue. The data on strong acids indicates that a third protective mechanism, buffering by hydroxyapatite, can contribute to dentine's protective effect in situations where the potential toxin is strongly acid. There may be other mechanisms by which dentine protects the pulp against chemical toxins from restorative materials. However, the three described: diffusion limitation; limited wetness for hydrolysis; and buffering by dentinal hydroxyapatite, appear to allow the relatively safe use of a wide range of tooth restorative materials.