SUMMARYCulture studies have suggested that Mycohacterium paratuherculosis m-ay p\ay a role in the aetiology ofCrohn's disease. However, evidence of sensitizalion to mycobacterial antigens amongst patients with Crohn's disease has not yet heen adequately demonstrated. Previous studies of cell-mediated iinmunity ((.'MI) in Crohn's disease were restricted lu responses of peripheral hlood numonuclear cells (PBMC) lomycohacterialanligcns. In this study we have investigated the proliferativc responses of hoth PBMC and mesenteric lymph node mononuclear cells (MLNMC) to a range of myeobaeterial and non-mycobacterial antigens. There was no evidence of speeifie sensitization in the responses of MLNMC and PBMC from patients with inflammatory howel disease (IBD) to the myeobaeterial antigens. However, anergy to A/./«;r(//((/)crcH/o.v(.scould not be excluded. IBD MLNMC responses to most antigens were generally greater than those of PBMC. which were often undetectable. When compared with controls, there was evidenee of increased CMI to a range of non-mycobacterial antigens, especially Yersinia enterocolitica, amongst both MLNMC and PBMC from patients with Crohn's disease and ulccrative colitis (IJC). These results do not provide support to the proposed role of mycobacteria in the ptithogenesis ofCrohn's disease, but indicate that further investigation may determine a role for bacterial-specific T cell-mediated responses in the pathogenesis of IBD.