Agglutinins to four strains of anaerobic gram-positive coccoid rods (species of Eubacterium, Peptostreptococcus and Coprococcus) were found in significantly higher frequency in Crohn's disease (CD) than in ulcerative colitis (UC) and in other diseased control subjects and were virtually absent in apparently healthy subjects. When the posterior probability of having CD was calculated on the basis of these agglutination reactions, 64% of patients with CD and 34% of patients with UC but only 10% of diseased controls and none of the healthy controls were regarded as 'probable' or 'definite' cases of CD. However, the posterior probability of CD did not sharply differentiate between CD and UC but indicated chronic inflammatory bowel disease. Factors contributing to the appearance of these agglutinins in CD were also evaluated. The findings would indicate the importance of a damaged intestinal mucosal barrier for the production of these agglutinins, provided the antigens are present in the intestine. No significant differences were observed between the occurrence of antibodies to pseudomonas-like organisms (PLO) in CD and the various control groups. The study could not add further evidence to the hypothesis of a possible aetiopathogenic role of PLO in CD.