Hamster flora introduced into germfree mice reduced the cecum to conventional size, suppressed populations of Escherichia coli and Clostridium difficile to the same degree that mouse flora did, and corrected the hypocellularity that is characteristic of the small bowel of germfree mice. A highly toxigenic strain of C. difficile readily induced cecitis in germfree and antibiotic-treated conventional mice, and histological examination frequently revealed pseudomembranes. Toxins A and B were both detected in ceca of animals with colitis. Gnotobiotic mice provide a model in which to study the role of the indigenous microflora in protecting against antibiotic-associated colitis.