Antibodies to folate-binding protein (FBP) of cow's milk were studied in serum from patients with chronic inflammatory bowel disease (CIBD), since earlier studies had shown IgG- and IgM-serum antibodies to other isotypes of milk proteins to be elevated in such patients, indicating a possible primary role or an epiphenomenon in the pathogenesis of CIBD. The amount of IgG antibodies to FBP was compared to disease activity, localization and duration by means of a newly developed enzyme-linked immunosorbent assay (ELISA). No statistically significant differences were seen between groups of patients with ulcerative colitis or Crohn's disease, as compared to healthy volunteers. Median values of FBP for ulcerative colitis were: 13 units/ml (4-74), for Crohn's disease: 14 units/ml (7-73), and healthy volunteers: 14 units/ml (4-51). No correlation was seen when comparing serum antibody values to disease activity, localization or duration. The intra-assay coefficient of variation of the ELISA technique was 0.05. The concentration of FBP in cow's milk (10 mg/l) is more than 50 times lower than that of other proteins (albumin, lactoglobulin, and casein). Even if a local defect in the endothelial barrier in these diseases may allow access of other milk protein antigens into the blood circulation, FBP is consequently not of importance for the humoral immune response in CIBD.