Objective: A limitation of antibiotic treatment is that even after successful killing of the infecting organism, secreted virulence factors may still be present and cause significant damage to host tissues. Photodynamic therapy is a promising new strategy for the treatment of periodontal disease; therefore if light-activated antimicrobial agents can also inactivate microbial virulence factors, this would represent an advantage of photodynamic therapy over conventional antibiotic treatment. Porphyromonas gingivalis is regarded as one of the major pathogens associated with advancing periodontitis and elaborates a number of virulence factors that contribute to the pathology of the disease. In this study, the ability of the light-activated antimicrobial agents methylene blue and tin chlorin e6 to inhibit the activity of proteases and lipopolysaccharide produced by P. gingivalis was investigated. Methods: Virulence factors were exposed to 665 nm or 633 nm laser light in the presence of methylene blue or tin chlorin e6 respectively and their activities determined. Results: The proteolytic activity of P. gingivalis was shown to be significantly reduced in a dose-dependent manner by treatment with both methylene blue and tin chlorin e6 in the presence of laser light; however the biological activity of P. gingivalis lipopolysaccharide was found to be inhibited by methylene blue and laser light of 665 nm only. Conclusion: These results suggest that in addition to killing P. gingivalis, photodynamic therapy may also be able to inactivate virulence factors produced by the pathogen, thereby reducing damage to the host and aiding the healing process.