Bacteroides fragilis is a member of the normal colonic microflora of most mammals and is the anaerobe most commonly isolated from human clinical specimens. It is associated with abscesses, soft-tissue infections, and bacteremias [1]. In the mid 1980s, Myers et al. [la] reported that some strains of B. fragilis produced a toxin that could be detected in ligated ileal loops of lambs. Since then, enterotoxigenic B. fragilis strains have been implicated as the cause of diarrhea in calves, piglets, foals, rabbits, and, more recently, humans [2]. The results from these studies together support the role of enterotoxigenic B. fragilis in gastrointestinal diseases in humans and animals. In 1992, Weikel et al. [3] showed that culture filtrates from the enterotoxigenic strains caused rapid morphological changes on human colon carcinoma cell lines, particularly HT-29, which are characterized by cell rounding and dissociation of F-actin. Furthermore, in 1996, Donelli et al. [4] showed that B. fragilis toxin-treated HT-29 cells demonstrated membrane blebbing and reorganization of their F-actin structure. We have purified the toxin (fragilysin) from culture supematants of enterotoxigenic B. fragilis. This toxin is a small polypeptide (~20,600 molecular weight) that causes fluid secretion in intestinal loops at 10 /g/mL and rounding of HT-29 cells at -10 ng/mL [5, 6]. Recently, in collaboration with Wells et al. [7], we showed that fragilysin increases bacterial intemalization and modulates the epithelial permeability of HT-29 enterocytes. Further studies at our laboratory have confirmed that fragilysin affects the tight junctions of epithelial cells (authors' unpublished data). These data suggest that fragilysin may disrupt the epithelial paracellular barrier through proteolytic degradation. The degradation of these extracellular proteins would then allow for cytoskeletal rearrangements because these proteins are directly linked through the cytoplasmic membrane [8]. We sequenced peptides obtained by proteolytic and chemical digestion of fragilysin and obtained sequences that were then used to generate primers for single specific primer (SSP)-PCR [9]. This process generated a fragment that contained a match to a consensus motif with zinc-metalloproteinases, especially within the metzincin superfamily. Further studies show that fragilysin has proteolytic activity. Fragilysin degrades a number of cellular proteins including fibrinogen, human complement C3, cytoskeletal proteins and some extracellular matrix proteins such as collagen IV [9]. We found that the purified toxin causes fluid secretion and exfoliation of the surface epithelial cells in vivo, and we have demonstrated that the proteolytic activity of the toxin is responsible for this effect [6]. Metalloproteinases have emerged as important virulence factors in a number of diverse pathogenic organisms including bacteria