ELISA and immunofluorescence tests were carried out on sera from patients with Crohn's disease and ulcerative colitis. The results indicate that mean antibody levels to Yersinia enterocolitica types O:9 and O:3 and Klebsiella pneumoniae, but not Pseudomonas maltophilia, are significantly higher in these patients than in control subjects. There are differing antibody levels in ileal and colonic disease, and there is a significant fall in antibody levels 9-12 months after remission from clinical symptoms in patients providing serial serum samples. However, there is no correlation between the severity of the disease and antibody titres.