Bacteria and their products stimulate inflammatory responses and may play a role in the pathogenesis of inflammatory bowel diseases. We investigated interactions between fecal flora and injured colonic mucosa using the trinitrobenzenesulfonic acid model of colitis in the rat. First, we tested the effects of broad-spectrum antibiotics administered at different times after induction of colitis. Second, in rats pretreated with antibiotics, we studied the effects of reintroduction of fecal flora on the release of inflammatory mediators and on the morphology of colonic lesions. Antibiotics reduced colonic lesion scores when started on day 1 after induction of colitis, but had no significant effect when started on day 7. In antibiotic-treated rats, intracolonic administration of a suspension of fecal contents 6 h after induction of colitis enhanced mucosal inflammatory activity and aggravated lesion scores. This effect was not observed when the suspension was administered 3 days after induction. Sonication of the suspension inhibited bacterial growth in aerobic and anaerobic cultures, and abolished its effect on colonic inflammation. In this model, bacteria of the common flora stimulate inflammatory activity and play a role in the induction of colonic mucosal lesions.