Houghton Poultry Research Station, Houghton, Huntingdon

baby rabbits have been used fairly extensively in studying the diarrhoea-producing effect of orally administered cell-free products of Vibrio cholerae. Essentially the same technique, but with piglets, has been employed by Kohler (1968) and Smith and Gyles (1970a) in studying porcine enteropathogenic strains of Escherichia coli. Exploiting the fact that the enterotoxin produced by these porcine strains is controlled by a transmissible plasmid designated Ent (Smith and Halls, 1968), Smith and Gyles ( 1 9 7 0 ~ ) were able to compare materials from strains that differed only according to whether they were Ent+ or Ent-. The value of such a well-controlled system became apparent when it was noted that under some conditions material from an Ent- strain could cause diarrhoea. Recently Smith and Linggood (1971) transmitted an Ent plasmid from a human enteropathogenic strain of E. coli to E. coli K12F-. Because the enterotoxin produced by this strain, like that produced by most other human enteropathogenic strains, is not active on ligated pig intestine, but is active on ligated rabbit intestine, it was decided to determine its effect when administere