Changes in the colonic mucosa-associated microflora were determined both in patients with active and inactive ulcerative colitis and in rats with acetic acid-induced colitis. In patients with active ulcerative colitis, significant decreases in the number of anaerobic bacteria (Brain Heart Infusion medium), anaerobic gram-negatives and Lactobacillus were found, whereas no changes were seen in the number of aerobic bacteria and Enterobacteriaceae. In patients with inactive ulcerative colitis, no significant differences in colonic mucosa-associated microflora could be demonstrated. Similar changes were seen in rats with acetic acid-induced colitis. Thus, 4 days after acetic acid administration, at which time the colitis was well developed as evaluated by morphological appearance and myeloperoxidase activity, reduction in the number of anaerobic bacteria and Lactobacillus was seen. The first day after acetic acid administration, when the colitis had not developed, or after 14 days, when the colitis had been overcome, no alterations were seen in the mucosa-associated microflora as compared with control rats. We conclude that a reduction in the number of anaerobic bacteria and Lactobacillus is a common feature in active colitis regardless of the origin of colitis.