In 18 patients with Crohn's disease, 28 patients with ulcerative colitis, 24 patients with enteritis and in 68 healthy adults lipid A antibody titers were determined by the passive hemolysis test. In addition, 0 antibody titers to polyvalent E. coli, Klebisella, Proteus, and Ps. aeruginosa antigens were measured by indirect hemagglutination. The patients with Crohn's disease showed a statistically significant elevation of the lipid A antibody titers compared with each of the three other groups investigated. The 0 antibody titers for the four polyvalent antigens were also higher in the patients with Crohn's disease than in the other groups. The results indicate that Enterobacteriaceae are involved in the pathogenesis of Crohn's disease. Thus long-term treatment with wide spectrum antibiotics seems to be justified. The determination of lipid A antibody titers may be a useful diagnostic tool in differentiating between Crohn's disease and ulcerative colitis.