A chronic colitis has been induced in rabbits having many of the histological features of human ulcerative colitis. Animals were first immunised with the common enterobacterial antigen of Kunin and haemagglutinating antibodies demonstrated in high titre. An immune complex colitis was then established by the injection of soluble immune complexes following mild irritation of the rectum with dilute formalin as previously described. The rabbits developed an acute colitis within the first week but, in contrast with unsensitised rabbits, the inflammation persisted and was still present at six months as assessed by proctoscopy and rectal biopsy. Kunin-sensitised rabbits receiving intravenous saline, antigen, or antibody alone did not develop a chronic colitis. It is suggested that hypersensitivity to colonic bacterial antigens may be one mechanism whereby an acute colitis becomes chronic.