Nine marrow allograft recipients who developed GVHD of the gastrointestinal tract accompanied by the passage of ropey necrotic material per rectum were studied. The material, resembling necrotic intestinal mucosa, was evaluated for the presence of epithelial cells using monoclonal antibodies for keratin and macrophages. Two keratins (AE1/AE3 and 34 beta E12) were detected within cells in all cases while macrophages were found in all but one case. In three cases, some cells were positive for both antibodies suggesting the presence of phagocytosed keratin in macrophage cytoplasm. Search for organisms with Gram and methenamine silver stains showed bacteria in six cases and fungus in one. Some cases of severe GVHD will be associated with the passage of ropey tan material grossly resembling sloughed mucosal tissue, but microscopically consisting largely of fibrin clots. The presence of free intestinal epithelial cells in this material is confirmed by the present study.