Arg-gingipain (Rgp) is a major cysteine proteinase produced by the oral bacterium Porphyromonas gingivalis, which is a major pathogen of advanced periodontal diseases. This enzyme is important for the bacterium both to exhibit its virulence and to survive in periodontal pockets. The development of Rgp inhibitors thus provides new therapeutic approaches to periodontal diseases. In this study, we first isolated and purified a novel and potent inhibitor of Rgp from the culture supernatant of Streptomyces species strain FA-70, now designated as FA-70C1. This compound was found to be an antipain analog composed of phenylalanyl-ureido-citrullinyl-valinyl-cycloarginal (C27H43N9O7). The Ki value was calculated to be 4.5x10(-9) M when benzyloxycarbonyl-phenylalanyl-arginine-4-methly-coumaryl-7-amide was used as a substrate. This compound also inhibited cathepsins B, L, and H, though their Ki values were much higher than that of Rgp. FA-70C1 had little or no inhibitory activity on Lys-gingipain, another cysteine proteinase of P. gingivalis. The Rgp-induced degradation of various human proteins was completely blocked by this inhibitor. Disruption of both the bactericidal activity of polymorphonuclear leukocytes and the viability of human fibroblasts and umbilical vein endothelial cells induced by the culture supernatant of P. gingivalis was suppressed by the inhibitor in a dose-dependent manner. The enhancement of vascular permeability induced by in vivo administration of the culture supernatant of P. gingivalis was strongly inhibited by the inhibitor. Furthermore, the growth of P. gingivalis was suppressed by FA-70C1 in a dose-dependent manner. These results strongly suggest that FA-70C1 is a useful tool to prevent the virulence of P. gingivalis.