SUMMARYThe morphology of antibiotic-associated enterocolitis in the hamster is described and compared with human antibiotic-associated pseudomembranous colitis. It is shown to be a caecal disease with proliferative mucosal changes and in this respect unlike the human counterpart. The bacteriology and toxicology, however, are identical. In addition, mucosal changes are described in animals on antibiotics but without established enterocolitis. As a result we suggest that there may be a spectrum of human disease ranging from mild antibiotic-associated diarrhoea to established pseudomembranous colitis. Therefore, despite the morphological variation, the hamster remains a good model for investigating the pathogenesis of pseudomembranous colitis and antibiotic-associated enteropathy in general. al., 1978b;Larson et al., 1978) since then have confirmed these findings and demonstrated cross-neutralisation between the toxin of C. difficile and C. sordellii antitoxin (George et al., 1978a). It seems established that proliferation of toxigenic strains of C. difficile in the human bowel is one mechanism in the pathogenesis of pseudomembranous colitis.In parallel with the findings in humans similar results have been produced in Syrian golden hamsters. Small (1968) initially demonstrated that lincomycin produced a fatal enterocolitis when given to hamsters. Because lincomycin and the related clindamycin have been the antibiotics at the centre of a spate of reports on antibiotic-associated pseudomembranous colitis (Cohen et al., 1973;Scott et al.,