It has been proposed that the presence of elevated serum titres against various Bacteroides species among patients with chronic inflammatory bowel disease (CIBD) may yield aetiological, pathogenic, or prognostic information. Using a crossed immunoelectrophoretic method, we investigated circulating antibodies against four Bacteroides species in 122 patients with CIBD (80 with ulcerative colitis (UC) and 42 with Crohn's disease (CD)) and in 32 patients with the irritable colon syndrome. In this cross-sectional study we found raised titre scores (greater than 0) among 26% of the patients with CD, among 46% of the patients with UC, and among 34% of the patients with the irritable colon syndrome. These differences are not significant. There were no correlations between the antibody titres and the duration of the disease, the clinical disease activity, or the site of the disease. Furthermore, the antibody titres yielded no prognostic information as to the necessity of surgery--that is, colectomy and/or small-bowel resection.