The behaviour of some serological aspects of autoimmunity was studied in patients with inflammatory bowel disease (IBD) by determining the incidence and levels of serum IgM, IgG and IgA rheumatoid factors (RF) and of circulating immune-complexes (IC) in 67 patients suffering from ulcerative colitis (UC) and 53 from Crohn's disease (CD). Sera from 40 young healthy subjects were included as controls. RF were detected by an enzyme linked immuno-assay (ELISA) and IC by the CIC-conglutinin and the CIC-C1q assays. ELISA detected a higher incidence of IgM RF in both UC and CD groups than the routine agglutination tests, although the mean levels were non significantly higher than in controls. Mean IgG RF levels were significantly above normal in UC and CD patients, the latter showing a greater incidence of elevated values and higher mean levels. IgA RF were significantly elevated only in CD patients; mean values for the UC group were within the normal range. The CIC-conglutinin test did not detect a significant incidence or high mean levels of IC and the CIC-C1q assay turned up a sizeable number of positive samples only in the CD group. Although still only preliminary, these overall findings suggest that RF activity is somehow present in patients with IBD. The greater prevalence of IC in CD patients, together with the patterns of IgG and IgA RF support the concept that this disease possibly involves autoimmune phenomena more than ulcerative colitis.