Five New Zealand White rabbits were injected intracolonically with homogenates (100 mum) of ulcerative-colitis tissue. Histological changes closely similar to those seen in the human donors were present in the mucosa and submucosa of the large intestine of four of these rabbits 3-12 months later. Similar changes were seen in the large intestine of three of four rabbits 6-13 months after intravenous or intracolonic inoculation of homogenates of rabbit mesenteric lymph-nodes after passage of human ulcerative colitis tissue (100 mum or 0.2 mum). Three of thirty A2G strain mice injected with similar tissue homogenates (100 mum or 0.2 mum) from patients with ulcerative colitis into the footpad or intraperitoneally, had granulomatous changes in footpad, bowel, liver and/or spleen 3-22 months later. Such changes did not develop in rabbits or mice inoculated with tissue from normal controls. The results of these experiments suggest that a transmissible factor is involved in the aetiology of ulcerative colitis.