A prospective study of faecal colonization with P-fimbriated Escherichia coli between 0 and 18 months of age was conducted in 751 healthy infants. The influence of breast-feeding and treatment with antibiotics on this colonization was studied. Colonization with P-fimbriated E. coli increased with age from 10% at 6 days to 30% at 18 months of age (P less than 0.01). Breast-feeding influenced colonization at 6 weeks of age when breast-fed children harboured fewer bacterial species (P less than 0.001) and fewer P-fimbriated E. coli (P = 0.06) than bottle-fed infants. Treatment with antibiotics increased the colonization rate with P-fimbriated E. coli at the age of 11 months (P less than 0.05). However, this was not true for treatment with ampicillin, which increased colonization rate with Gram-negative species other than E. coli (P less than 0.05). Fifty per cent (378) of all children were colonized and a quarter (183) had pure cultures of P-fimbriated E. coli in at least one faecal sample. The clinical importance of this colonization remains to be shown.