Attempts were made to discover the source of strains of haemolytic Escherichia coli infecting weaned pigs on a piggery. The organisms were not detected in the faeces of sows in the farrowing house, or in the in the faeces or intestinal tracts of slaughtered bacon pigs or sows. Sows held in a quarantine unit, and their offspring born in the unit, did not excrete haemolytic E. coli until after they were returned to the piggery. The environment of the piggery was the most likely source of infection for weaned pigs, and routine cleaning and disinfection of the accommodation did not prevent infection. Unweaned pigs were however able to transfer haemolytic E. coli to a newly built, previously unused weaning house, and establish a cycle of infection.