In patient with Crohn's disease antibodies against microorganisms of the intestinal flora are found more frequently than in healthy individuals. This probably reflects the increased immunoreactivity of the compromised intestinum. Antibodies against Saccharomyces cerevisiae are frequently found in the serum of patients suffereing from Crohn's disease. They enable the diagnostic differentation of Crohn's disease and ulceration colitis. Antibodies against Saccharomyces cerevisiae enrich the serological diagnosis of chronic inflammatory bowel disease by a further parameter, in addition to autoantibodies against exocrine pancrease, intestinal goblet cells and granulocytes (pANCA).