Qualitative and quantitative anaerobic cultures were performed on faecal samples from 27 normal full-term newborn infants; from 32 preterm infants during intensive or intermediate care, not treated with antibiotics; and from 106 mostly preterm newborns, treated with antibiotics for various reasons. There were no major differences between the children in the first two groups. In these, Caesarean section led to a lower isolation rate of bifidobacteria and a much lower incidence of Bacteroides spp. During antibiotic treatment anaerobic bacteria were isolated from only 10% of the infants. After treatment, there was a slow regrowth of bifidobacteria, but Bacteroides spp. were not usually re-established. There was a colonisation of infants delivered by Caesarean section with new Lactobacillus spp. after treatment. In particular Bacteroides colonisation may be facilitated and more stable if it occurs during passage through the birth canal.