Background: Citrullination of proteins within inflamed periodontal tissues may provide an important link between periodontitis and rheumatoid arthritis. The aim of this study is to determine whether the presence of Porphyromonas gingivalis peptidylarginine deiminase (PPAD) can influence citrullination of proteins by either increasing the amount of local citrullinated protein or influencing the peptidylarginine deiminase (PAD) enzymes found in the monocyte/ macrophage population. Methods: Human peripheral blood monocytes and macrophages were incubated in the presence of live or heatkilled P. gingivalis. Expression of PAD2 and PAD4, PPAD, and citrullinated proteins were assessed by either a combination of real-time polymerase chain reaction, Western blotting, or a colorimetric assay. Results: PPAD was detected only in mononuclear cells incubated in the presence of live P. gingivalis and resulted in increased extracellular citrullination. Endogenous PAD (mRNA and protein) expression was detected in monocytes and macrophages but was not affected by P. gingivalis. Conclusion: Although P. gingivalis produces a PAD that can citrullinate extracellular proteins and may contribute to the citrullinated protein load in gingival tissues, it does not appear to affect PAD expression or citrullination by host monocytes or macrophages.