Rectal biopsies were examined from 22 patients with Salmonella infection of food-poisoning type and from seven patients with inflammatory bowel disease and coincidental Salmonella infection. In the former group the changes observed were mucosal oedema with acute inflammation of varying severity but with preservation of the crypt architecture. Crypt abscesses were present in a few cases but were usually localized in the crypt and mucus depletion only occurred with severe inflammation. These features are not specific and are similar to those seen in other types of infective colitis such as Shigella dysentery, gonococcal proctitis and amoebic colitis. In the majority of cases of infective colitis the appearances are usually sufficiently distinctive, however, to distinguish them from those seen in ulcerative colitis and Crohn's disease. The changes in the biopsies from the seven patients with coincidental Salmonella infection were in general those of the underlying idiopathic inflammatory bowel disease.