Agglutinins to certain species of Eubacterium and Peptostreptococcus have been reported in sera from a high proportion of patients with Crohn's disease. Because this might be a non-specific finding common to patients with diarrhoea associated with damaged intestinal mucosa, we have compared the incidence of such agglutinins in patients with Crohn's disease with that seen in patients in North-East India with acute or chronic diarrhoea. The incidence of agglutinins in Crohn's disease was 44%, compared with 11% in acute and 17% in chronic diarrhoea. These figures suggest that mucosal damage alone does not explain the high incidence of agglutinins in Crohn's disease.