The role of bacteria and microbial agents in the pathogenesis of Crohn's disease has been suggested and is supported by animal models of inflammatory bowel disease (IBD) in which the presence of normal gut flora is required for the initiation and full expression of the inflammatory reaction. In human Crohn's disease, antibiotics are commonly used even though the evidence to support this practice is not strong. Several studies have indicated that antibiotics have some biological activity in Crohn's disease and that this activity may be most pronounced in the subset of patients with colonic involvement.