Plaque-induced inflammation has been documented from various aspects, but is still not well understood. In an effort to clarify the physicochemical properties of dental plaque and to isolate the active factor which give rise to inflammation, we studied a factor in plaque which may be a direct and local cause of gingival inflammation in ODU rats. Rat dental plaque was dissolved in physiologic saline. After being homogenized, the solution was centrifuged and the supernatant (RP) was used as a sample. Protein component (pRP) of PR was isolated by ammonium sulfate fractionated procedure and then deproteinized RP (dpRP) was obtained by trichloroacetic acid treatment. The vascular permeability factor existed in RP and it was affected by heating and pH. Changes in vascular permeability were directly proportionate to the logarithm of the concentration of protein fraction in dental plaque, and were of a delayed type. It was surmised that pRP contained in RP was the main vascular permeability factor, and it was purified by gel filtration on Sephadex G-100 and G-200.