In order to evaluate the role of mycobacteria in the pathogenesis of Crohn's disease (CD), 4 monoclonal antibodies (McAb's) raised against M. avium specific glycolipid were tested on bowel resection specimens of CD patients and relevant controls. Two of the McAb's had shown a positive reaction to a CD-Mycobacteria isolate (CD-Myc). The same two McAb's showed a positive reaction within the gut wall, not only in CD patients, but also in the controls. In non-CD controls, however, the positive cells were limited to the lamina propria, while in CD patients positivity was also found in the submucosa and subserosa. Furthermore the mean number of positive cells in CD patients tends to be higher than in the control groups. Using double staining techniques the positive cells appeared to be B-cells of IgA isotype. These preliminary results indicate that mycobacteria might play a role in the pathogenesis of Crohn's disease.