One hundred and sixty patients have been studied to assess the significance of circulating antigen-antibody complexes in inflammatory bowel disease (IBD). Sera from patients with ulcerative colitis and Crohn's disease were assayed for Clq-binding antigen-antibody complexes (CIC) and the results compared with those obtained from 52 patients with other inflammatory gastrointestinal diseases (GI controls) and 26 normal volunteers (normal controls). Significantly elevated CIC levels were found in patients with active IBD compared with inactive IBD or normal controls. However, similar high levels were also found in the GI control group. CIC levels in a group of undernourished patients with Crohn's disease fell significantly after treatment with an oral nutritional supplement. The results suggest that CIC-mediated inflammation is likely to be associated with intestinal mucosal disease in a non-specific manner.