Strains of Escherichia coli K12F- to which Ent plasmids from a human and a porcine enteropathogenic strain of E. coli had been transmitted yielded cellfree preparations that produced diarrhoea when administered orally to baby rabbits. The preparations obtained from the strain containing the “porcine” plasmid was more active in this respect than were those of the strain containing the “human” plasmid. This difference was also observed when preparations of wild human and porcine enteropathogenic strains were compared. The diarrhoea produced in the baby rabbits resembled that produced by Vibrio cholerae enterotoxin.