Interestingly, using a monoclonal antibody, peptidoglycan-polysaccharide complexes (PPC) were detected intracellularly in the mucosa and submucosa of the bowel wall of Crohn's disease (CD) patients. PPC are the main constituents of the gram-positive bacterial cell wall. These PPC were however detected in the normal bowel wall also. Therefore, in this study the hypothesis that an enhanced immune responsiveness to bacterial antigens plays a pivotal role in the induction or the chronicity of CD was tested. As antigens, the peptidoglycan structures of intestinal bacteria (Eubacterium aerofaciens or fecal PPC) or of Streptococcus pyogenes, the 65-kDa heat shock protein and muramyl dipeptide (MDP), the smallest bioactive subunit of peptidoglycan, were used. The proliferative responses of peripheral blood (PB) mononuclear cells (MNC) of healthy subjects and patients in a remissive stage of CD or an active CD stage were examined. Of this last patient group the MNC responses of the mesenterial lymph nodes that drain the inflamed gut area were measured also. The responses of PB-MNC of the healthy subjects and the patients in a remissive CD stage were not different. Compared to the responses in remissive CD, the PB-MNC responses in active CD to the eubacterial cell wall and streptococcal cell wall antigen were significantly higher. At the inflammation site in active CD, the lymph nodes, the responses to most of the bacterial antigens were significantly higher than in the PB. In summary, the results show the presence of bacterial peptidoglycan in the bowel wall and the immune responsiveness, especially at the inflammation site, to these antigens in active CD and therefore present suggestive evidence for the role of peptidoglycan in the etiology and/or pathogenesis of CD.