Lipid A antibody titers and O antibody titers against E. coli were determined in 18 patients with Crohn's disease, 28 patients with ulcerative colitis, 24 patients with acute enteritis and in 68 healthy adults. The patients with Crohn's disease showed a statistically significant elevation of the lipid A antibody titers and of the O antibody titers against E. coli compared with each of the three other groups investigated. The results could be indicative of bacterial involvement in the pathogenesis of Crohn's disease. The determination of lipid A antibody titers may be useful for the differential diagnosis between Crohn's disease and ulcerative colitis.