Faecal samples were collected from 23 patients with active ulcerative colitis, 15 patients with established ulcerative colitis in remission, 20 patients with active colitis of cause other than ulcerative colitis, and 20 normal control subjects. Ten coliform colonies were randomly selected from the faecal sample cultures and serotyped before the testing of each different serotype from each sample for adhesive or invasive properties on HeLa cell monolayers. In the patients with both active ulcerative colitis and ulcerative colitis in remission and those with other types of colitis one serotype tended to dominate the faecal coliform flora. In normal controls more variety was encountered. Thirty-five per cent of the patients with active ulcerative colitis and 27% of the patients with ulcerative colitis in remission had at least one adhesive or invasive faecal coliform as compared with 5% of the patients with other types of colitis and 5% of the normal controls. These findings are significant (P < 0.05) and may have aetiological and therapeutic significance.