Ten piglets and four monkeys free from Balantidium were dosed with human faecal homogenate which contained 1.2 x 10(4)-4.8 x 10(4) B. coli cysts. The infection resulted in severe diarrhoea in piglets 1-6 and hydrocortisone-treated monkeys 1-2, moderate diarrhoea in piglets 7-10 and a subclinical infection in monkeys 3-4. In piglets 1-3 and monkeys 1-2, heavy infection of the intestinal mucosa extended from the terminal ileum to the rectum and the mucosa was severely damaged. In piglets 4-10, infection was heavy in the large intestine with moderate mucosal damage.