After exposure to a variant of Citrobacter freundii, suckling and adult mice developed transmissible murine colonic hyperplasia of the same degree of severity. Mucosal hyperplasia was most severe 2 to 3 weeks after inoculation and then regressed. Suckling mice had a high mortality because of secondary inflammatory and erosive changes. Severe hyperplasia was characterized by mitotic activity along the entire crypt column and surface mucosa.