The cloned Porphyromonas gingivalis alkyl hydroperoxide reductase (ahpC) gene complemented an ahpC defect in Escherichia coli. To study the role of ahpC in protecting against oxidative stress in P. gingivalis a 1.8 kb fragment containing the ahpC gene was amplified from the chromosome of P. gingivalis W83. This gene was insertionally inactivated using the ermF-ermAM antibiotic resistance cassette and used to create a ahpC-deficient mutant by allelic exchange. One mutant strain, designated FLL141, demonstrated no change in the growth rate, black pigmentation, beta-hemolysis or level of proteolytic activity compared to the parent strain. Although P. gingivalis FLL141 was more sensitive to hydrogen peroxide than the parent strain, there was no change in its virulence potential in the mouse model compared to the wild-type strain. These findings suggest that the ahpC gene plays a role in peroxide resistance in P. gingivalis but does not contribute significantly to virulence.