ABSTRACT The contributions of three proteinase genes ( rgpA , rgpB , and kgp ) to the virulence of Porphyromonas gingivalis W50 were investigated in the murine periodontitis model. Mice were orally inoculated with eight doses (1 × 10 ¹⁰ cells per dose) of rgpA , rgpB , kgp , rgpA rgpB , or rgpA rgpB kgp isogenic mutants, and the level of alveolar bone loss, immune response induced, and number of bacterial cells per half maxilla were compared with those of animals inoculated with wild-type P. gingivalis . The kgp , rgpB , rgpA rgpB , and rgpA rgpB kgp isogenic mutants induced significantly ( P < 0.05) less bone loss than the rgpA isogenic mutant and the wild type did, and the virulence of the rgpA isogenic mutant and the wild type were not significantly different. Mice inoculated with the wild type or the rgpA isogenic mutant exhibited significantly ( P < 0.01) more P. gingivalis cells per half maxilla than mice inoculated with rgpB , kgp , rgpA rgpB , and rgpA rgpB kgp isogenic mutants or nonchallenged mice did, as determined using real-time PCR. A significant positive correlation was found between the number of P. gingivalis cells detected per half maxilla and the amount of alveolar bone loss induced. Enzyme-linked immunosorbent assay results showed that each isogenic mutant and the wild type induced a predominant P. gingivalis antigen-specific immunoglobulin G3 (IgG3) response. Furthermore, the kgp and rgpA rgpB kgp isogenic mutants induced significantly ( P < 0.05) lower IgG3 antibody responses than the responses induced by the wild type or the rgpA , rgpB , and rgpA rgpB isogenic mutants. The results suggest that the order in which the proteinases contribute to the virulence of P. gingivalis in the murine periodontitis model is Kgp ≥ RgpB ≫ RgpA.