The faecal flora of patients with Crohn's disease has been found to differ from that of healthy subjects in that the numbers of anaerobic gramnegative rods and of gram-positive coccoid anaerobes, belonging to species of Eubacterium and Peptostreptococcus were higher. The flora composition was independent of duration of illness and was not influenced by ileocaecal resection. Serum agglutinins against some strains of coccoid rods were found in a considerable percentage of patients with Crohn's disease, whereas percentages of positive sera were much lower in healthy subjects and in patients with various diseases. The interpretation of these data established by Wensinck and the use of the agglutination reactions as a diagnostic test are the subject of this thesis. In Chapter 2 recent microbiological and immunological findings in patients with Crohn's disease are reviewed. They show that in Crohn's disease as well as in other intestinal diseases, like ulcerative colitis, antibodies to dietary and microbial antigens are found frequently. In Chapter 3 results are presented of investigations on the prevalence of agglutinins to four strains of anaerobic coccoid rods in patients with Crohn's disease, ulcerative colitis, a number of other diseases and in healthy subjects. Antibodies to coccoid rods were found much more frequently in Crohn's disease than in ulcerative colitis and other diseases. Using the interpretation of agglutination reactions as described in Chapter 7, the percentage of false positive results of sera submitted for diagnosis was found to be satisfactorily low. The data in Chapter 4 show that the presence of antibodies to the coccoid rods in patients with Crohn's disease is correlated with colonic disease, the presence of fistulae and with serum immunoglobulin levels. No correlation was found between antibodies and any index of disease activity.