To determine whether circulating immune complexes are present in the sera of patients with inflammatory bowel disease (IBD), a 125I-Clq binding assay was performed. Of the 55 IBD serum samples tested, the 24 ulcerative colitis samples demonstrated significant binding (33.1 ± 8.3%, p = 0.02), whereas the 31 Crohn’s samples bound essentially normal amounts (29.2 ± 7.4%). A positive control group consisting of 27 patients with rheumatoid arthritis was also studied. Sera from 4 patients with IBD and colonic cancer when tested, bound 40.2 ± 8.0% of the available 125I-Clq, while 10 patients with previous colectomies and ileostomies gave results similar to those of 15 healthy controls and 11 patients with irritable c