The antimicrobial properties of cements prepared from mechanically activated tetracalcium phosphate (maTTCP) were tested with the agar diffusion test using Streptococcus salivarius, Staphylococcus epidermidis, and a clinically isolated plaque mixture. All maTTCP cements showed a significantly higher antimicrobial potency as revealed by inhibition zones of approximately 3-5 mm width, compared with a commercial Ca(OH)(2)//salicylate cement which only produced small inhibition zones around the cement specimens of 1.5 mm or less. This behavior was explained by the formation of amorphous Ca(OH)(2) during setting of maTTCP cements, which is thought to have a higher solubility and may release more OH(-) ions than conventional Ca(OH)(2)//salicylate cements. In fact, the pH value in the agar gel around the specimens was higher in the case of maTTCP cements (7.8-8.7) compared with the Ca(OH)(2)//salicylate control (7.0-8.0). The maTTCP cements did not affect the photoactivation of resin-based composites, and their antimicrobial activity is making them interesting candidates for the use as pulp-capping agents, endodontic sealers, or cavity liners in dentistry.