To clarify the role played by Bacteroides vulgatus in the pathogenesis of ulcerative colitis, the serum antibody titer to the outer membrane protein of B. vulgatus, obtained from the inflamed rectal mucosa, and measured by enzyme-linked immunosorbent assay, was investigated in patients with ulcerative colitis. High serum antibody titers to the outer membrane protein of B. vulgatus were found in these patients. The specific outer membrane protein of B. vulgatus reactive to serum antibody was recognized as a 26-kDa protein. These findings indicate that B. vulgatus containing a 26-kDa protein in the outer membrane may be implicated in the pathogenesis of ulcerative colitis.