Acute exacerbations of chronic inflammatory bowel disease (ulcerative colitis and Crohn's disease) are characterised by an increase in immunoglobulin G (IgG) positive cells in the mucosa, whereas uninflamed mucosa of inflammatory bowel disease patients displays only moderately increased or normal numbers of these cells. Previous data suggest that acute exacerbations of ulcerative colitis and Crohn's disease can be distinguished by different IgG subclass expression of mucosal immunocytes and a different IgG subclass production pattern of lamina propria lymphocytes. A procedure to obtain enough intestinal mononuclear cells from biopsy specimens to measure in vitro IgG and IgG1 production in control subjects and various patient groups has been established. IgG2 could be measured in Crohn's disease and ulcerative colitis only, as the concentrations in control subjects were below the sensitivity of the ELISA method. We found that IgG and IgG1 production correlated with the degree of local inflammation in both diseases, even in slightly inflamed mucosa, compared with control subjects. The proportion of IgG1 subclass was significantly increased in severely inflamed mucosa of both ulcerative colitis and Crohn's disease patients. A major difference between Crohn's disease and ulcerative colitis mucosa is apparent in mild or no inflammation. In Crohn's disease mucosa in remission, the IgG1/IgG ratio is comparable with that in controls, yet ulcerative colitis mucosa still displays significantly increased proportions of IgG1. In addition, the IgG2/IgG ratio is 0.12 in ulcerative colitis and 0.19 in Crohn's disease patients. The results show the dependence of local IgG and IgG1 production on the degree of inflammation and that an increase in subclass IgG1 in ulcerative colitis is present at all stages, including remission. These findings support the hypothesis that different immunoregulatory mechanisms are involved in Crohn's disease and ulcerative colitis. Environmental stimuli or genetic background may be responsible for the observed differences.