Circulating antibodies against human colonic extract enriched with a 40 kDa protein in patients with ulcerative colitis. We have previously described a 40 kDa colonic protein(s) which is specifically recognised by tissue-bound immunoglobulin G obtained from the colon of patients with ulcerative colitis. We now report the presence of circulating antibodies against this antigen using an enzyme-linked immunosorbent assay with a highly enriched preparation of the 40 kDa protein from normal colon extracts. Serum was collected from 79 patients with ulcerative colitis, 36 with Crohn's disease, 16 with specific diarrhoeal syndromes, and from 19 normal subjects. Twenty nine of 79 patients with ulcerative colitis, 21 of 36 with Crohn's disease, and all patients with diarrhoea were symptomatic during the collection of sera. The difference in optical density values between patients with symptomatic ulcerative colitis and each of the other groups, including patients with ulcerative colitis in remission, was highly significant (p less than 0.01). Seventy nine per cent of patients with symptomatic ulcerative colitis had optical density values above the means for all other groups. Fifty five per cent of sera from patients with symptomatic ulcerative colitis had optical densities beyond two SDs of the values for all other groups and only two of 71 sera from non-ulcerative colitis patients (one Crohn's disease and one normal subject) had values in this range. These results show the presence of anti-colon antibodies against the 40 kDa protein(s) in the sera of many patients with symptomatic ulcerative colitis. 