Treponema denticola is a major pathogen of chronic periodontitis. Analysis of the T. denticola genome revealed a gene orthologous with a cysteine protease-encoding gene from Streptococcus pyogenes (IdeS). IdeS interferes with IgG-dependent opsonophagocytosis by specific cleavage of IgG molecules. Analysis of this gene (termed ideT) revealed it to encode a two-domain protein whose N-terminus is composed of tandem immunoglobulin-like domains followed by a C-terminal IdeS-like protease domain. In this study we show that during secretion the IdeT protein is processed into an N-terminal fragment which remains associated with the cell, and a C-terminal part released into the medium. Although the secreted domain of IdeT, termed dentipain, shows only 25% identity to the IdeS protease, the putative catalytic cysteine and histidine residues are strongly conserved. Recombinant dentipain cleaves the insulin β-chain, an activity which is inhibited by E-64, a diagnostic inhibitor of cysteine proteases. Apart from insulin no cleavage of other protein substrates was detected, suggesting that dentipain has oligopeptidase activity. A mutant strain was constructed expressing a modified IdeT variant, the dentipain domain of which was deleted. This strain was found to be significantly reduced in its abscess-forming activity compared with the parental strain in a murine abscess model, suggesting that dentipain contributes to the virulence of T. denticola.