It is not entirely possible to prevent hospital cross-infection in infantile gastroenteritis by the use of conventional barrier-nursing techniques, even with experienced staff in well-designed wards. An incident of double cross-infection due to enteropathic Esch. coli types O119 and O126 and the measures that were taken to control it are described. The type O119 infections were clinically severe, and the cases showed significant levels of serum antibody to the organism, while the type O126 infections were clinically mild and no antibody was produced. The use of the antibiotic colistin, to which both organisms were originally sensitive, may have been of some value in clearing the type O119 infection but was without effect on the type O126 infection, which continued to spread during treatment. A degree of resistance to colistin was found in some strains of the type O126 organism isolated after treatment. The outbreaks largely ended spontaneously, but may have been limited in extent by the administrative measures described. The use of antibiotics in infantile gastroenteritis is reviewed and a plea is made for a modern large-scale trial to reassess their value.