Bacteroides vulgatus, isolated from a patient with Crohn's disease, produced in gnotobiotic rats 7 constitutive enzymes that might be concerned with the degradation of intestinal glycoproteins. Furthermore Bacteroides vulgatus caused an almost complete loss of blood group antigenicity of the intestinal glycoproteins. Enzymes with the potency to release toxic compounds from hepatic conjugates and plant glycosides, beta-glucuronidase and beta-glucosidase, respectively, were only detectable in small amounts. These findings indicate that Bacteroides vulgatus, which accounts for 40% of the total flora of patients with Crohn's disease, may play a role in the pathogenesis of Crohn's disease, by increasing the break-down of the mucus layer and therefore damaging its protective function.