We studied the class-specific antibody response to the cow's milk antigen beta-lactoglobulin (beta-LG) in sera from patients with ulcerative colitis and Crohn's disease. IgG and IgM to beta-LG were significantly higher in patients when compared to healthy non-atopic controls, whereas IgA values were similar, and specific IgE absent in all groups. No correlation between IgG- or IgM-containing immune complexes was found with the corresponding isotype of antibody to beta-LG; however, IgM complexes correlated with serum total IgM in ulcerative colitis. In these patients, IgG antibodies were higher in active cases, whereas IgM increased in patients without signs of disease activity. Antibody titers did not correlate with disease duration or administration of antiinflammatory drugs. This pattern of anti-beta-LG reactivity suggests that the presence of intestinal lesions may be revealed by the selective increase of some antibody isotypes to orally administered antigens. Enhanced mucosal permeability may be studied by this type of serological analysis.