The colonisation by enterobacteria of neonates, born normally, has been studied by quantitative and qualitative techniques (by the determination of the differing patterns of antibiotic resistance). The resistant enterobacteria were less common than the sensitive strains but resistant ones were widespread in the first week of life and 6 out of 10 babies became carriers of Klebsiella. The results enabled epidemiological observations to be made (the role of the mother, who only seems to be important during delivery, and the importance of the bacteria carried on the hands of the staff), and to assess the effect of antibiotics in hospital even if they are not given to the baby.