In this study, we demonstrate that Porphyromonas gingivalis fimbrillin, a major component of bacterial fimbriae, is one of the fibronectin-binding proteins and that fibronectin is a potent inhibitor of the adherence of the bacteria to host cells and of the pathogenesis of the bacterium that acts by binding to the fimbriae. A Western blotting (immunoblotting) assay showed that fibronectin binds strongly to P. gingivalis fimbrillin. The fimbrial binding to fibronectin was also evidenced by a binding assay involving 125I-labeled fimbriae. Furthermore, fibronectin markedly inhibited the fimbria-induced expression of interleukin-1beta and neutrophil-specific chemoattractant KC genes in macrophages. The inhibitory action depended on the fimbrial interaction with heparin-binding and cell attachment domains in the fibronectin structure. The binding of P.gingivalis to mouse peritoneal macrophages via its fimbriae was inhibited by fibronectin. Fibronectin also inhibited the bacterial cell-induced expression of interleukin-1beta and KC genes in the macrophages. These results demonstrate the importance of fibronectin as a modulator of the pathogenic mechanism of P. gingivalis, a pathogen that causes adult periodontal disease.