Serum antibodies of immunoglobulin G, immunoglobulin M, and immunoglobulin A isotypes to five major proteins of cow's milk, casein, bovine serum albumin, alpha-lactalbumin, beta-lactoglobulin A, and beta-lactoglobulin B, were measured using enzyme-linked immunosorbent assay in 51 patients with ulcerative colitis, 49 with Crohn's disease, and 20 age-matched controls. Immunoglobulin G and immunoglobulin M antibodies to cow's milk proteins were significantly elevated in patients with inflammatory bowel disease as compared to controls. In contrast, no significant increase in immunoglobulin A antibodies to 3 of 5 proteins was noted. The increased titers of antibodies to milk proteins seem to be specific and not due to a polyclonal immunoglobulin activation, as naturally occurring blood group antibodies were not elevated in patients with ulcerative colitis and Crohn's disease. A good correlation of disease activity, as measured by serum alpha 1-acid glycoprotein concentrations, and immunoglobulin G and immunoglobulin A antibody titers against certain cow's milk proteins could be demonstrated in Crohn's disease, but not ulcerative colitis. These findings suggest that production of antibodies to cow's milk proteins reflects specific immunization with these antigens. The study of antibody isotypes and correlation with disease activity may provide better insight into the immune response to dietary antigens and its possible role in the pathogenesis of inflammatory bowel diseases.