Abstract: This invention relates to the PrtR-PrtK cell surface protein of Porphyromonas gingivalis and in particular a multimeric cell association protein complex comprising the PrtR and PrtK proteins. Accordingly the invention provides a substantially purified antigenic complex for use in raising an antibody response directed against Porphyromonas gingivalis. The complex comprises at least one multimeric protein complex of arginine-specific and lysine-specific thiol endopeptidases each containing at least one adhesin domain, the complex having a molecular weight of greater than about 200 kDa. The invention also relates to pharmaceutical compositions and associated agents based on said complex for the detection, prevention and treatment of Periodontal disease associated with P. gingivalis.