The possibility of autoimmune type I reactions to cellular constituents was investigated in 22 patients with ulcerative colitis, 12 with Crohn's disease, and in 22 healthy volunteers. Nuclear components and colon mucosa fragments were tested as potential antigens by the basophil histamine release technique. One of 12 patients with ulcerative colitis responded to sonicated leukocyte nuclei and one of 12 patients with Crohn's disease responded to both nuclei and RNA. Increased serum levels of total IgE and antinuclear antibodies of IgE class were found in one and three of the 24 patients, respectively. Histamine release caused by colon mucosa fragments was not observed in a separate study consisting of 10 ulcerative colitis patients and 10 healthy volunteers. Autoimmune type I allergy to cellular constituents does not seem to be of significance for chronic inflammatory bowel disease and thus could not explain the involvement of tissue mast cells and eosinophils in this condition.