Porphyromonas gingivalis, a bacterium associated with active chronic periodontitis lesions, produces several proteolytic enzymes that are thought to be involved in host colonization, perturbation of the immune system, and tissue destruction. The aim of the present study was to investigate the contribution of Arg- and Lys-gingipains produced by P. gingivalis to its growth. Although all of the proteins studied were degraded by P. gingivalis, only human serum albumin and transferrin supported growth during serial transfers in a chemically defined medium (CDM). Growth studies with site-directed gingipain-deficient mutants of P. gingivalis revealed that inactivation of both gingipains prevents growth, whereas inactivation of either Arg- or Lys-gingipain activity extended the doubling times to 33 or 13 h, respectively, compared to 9 h for the parent strain. Growth of the mutants and the parent strain was similar when the CDM was supplemented with a protein hydrolysate instead of human serum albumin. Incubation of resting P. gingivalis ATCC 33277 cells with fluorophore-labeled albumin indicated that the proteolytic fragments generated by the gingipains were internalized by the bacterial cells. Internalization of fluorophore-labeled albumin fragments was reduced or completely inhibited in the proteinase-deficient mutants. Interestingly, gingival crevicular fluid samples from diseased periodontal sites contained low-molecular-mass albumin fragments, whereas samples from healthy sites did not. The critical role of proteinases in the growth of P. gingivalis was further investigated using specific Arg- and Lys-gingipain inhibitors. Adding the inhibitors to CDM containing albumin revealed that leupeptin (Arg-gingipain A and B inhibitor) was more efficient at inhibiting growth than cathepsin B inhibitor II (Lys-gingipain inhibitor). Our study suggests that Arg-gingipains and, to a lesser extent, Lys-gingipain play an important role in the growth of P. gingivalis in a defined medium containing a human protein as the sole carbon and nitrogen source.