The colonic biopsy specimens of 22 patients with colitis and positive stool cultures for Campylobacter jejuni were studied in order to obtain histological and immunohistochemical criteria to differentiate Campylobacter colitis from chronic inflammatory bowel disease. In addition we tried to identify Campylobacter inclusions by means of immunohistochemistry and electron microscopy as evidence for invasion of the colonic mucosa. The results show that the majority of patients with Campylobacter colitis have the histological picture of acute infectious colitis with increased numbers of IgA and IgM containing plasma cells in the colonic mucosa in contrast with patients with active chronic inflammatory bowel disease who show increases of IgA and IgG (ulcerative colitis) or IgA-, IgM and IgG containing plasma cells (M Crohn) in their colonic biopsies. The results of immunohistochemical stainings with Campylobacter antiserum show invasion of Campylobacter in the colonic mucosa. These findings were confirmed ultrastructurally.