To examine and compare the pathogenicity of cytotoxic necrotising factor (CNF)-producing Escherichia coli, two litters of piglets were infected orally with 10(10) E coli O88 or 10(10) E coli O32 strains. Of the six piglets infected with E coli O88, two died within 24 hours and three developed blood-stained diarrhoea. The other piglets were killed one, five, six and eight days after infection, when bacterial cultures indicated an overwhelming bacteraemic infection with E coli O88 in the early stages followed by clearance through the large intestine. The pathological changes consisted of an early enteritis, progressing to enterocolitis and a bacteraemic spread to the lungs. The histopathological changes were characteristic of toxaemic effects in brain, heart, liver and kidney, and characterised by congestion, oedema and exudation. Infection with E coli O32 produced a milder but similar enterocolitis, also with bacterial colonisation in the lungs. The histopathological findings again reflected a toxaemia. The enteritis was more persistent after E coli O32 infection and the strain persisted in large numbers in the intestine. No evidence of bacterial adherence to the intestinal mucosa was found with either strain. Enteroinvasion was only evident in one E coli O88-infected piglet, but the consistent occurrence of interstitial pneumonia showed the predilection of these organisms for the lung. The results confirm the toxigenic properties of CNF+ E coli and suggest an important role for this organism in enteric infection of young pigs.