Porphyromonas gingivalis has been closely associated with the initiation and progression of some forms of periodontal diseases and its proteolytic enzymes have been implicated in invasion, tissue destruction and evasion of host antibacterial defenses. Recently, the primary focus of research has been on cysteine proteinases, referred to as gingipain R and gingipain K which are produced in large quantities and are directly involved in pathological events during development and progression of periodontitis, contributing to clinical hallmarks of the disease including: flow of gingival crevicular fluid, neutrophil accumulation and bleeding on probing. Gingipain R exists as 110-, 95-, 70- to 90- and 50-kDa proteins, the first two being a complex of the 50-kDa catalytic subunit with hemagglutinin/adhesins, with or without an added membrane anchorage peptide. The other forms are single-chain enzymes. The predominant form of gingipain K in P. gingivalis strains is a complex of a 60-kDa catalytic protein with hemagglutinin/adhesins. Molecular cloning and structural characterization of the gingipain R and gingipain K genes has shown that they code for 1704 and 1722 amino-acid residue preproenzymes, respectively. Although both structures show no similarity within the preprofragment and only limited identity within the catalytic domain (27%) they are essentially identical within the putative hemagglutinin/adhesin domain. Furthermore, on the basis of gene structure it is now apparent that various soluble and membrane bound forms of gingipains are derived through proteolytic processing of the preproenzymes, and it can be assumed that the Arg-X-specific enzyme is responsible for this processing.