To study the effect of milk supplemented with iron on neonatal gut flora, faecal specimens of ten infants receiving breast milk, six receiving a cow-milk preparation supplemented with iron (5 mg/l) and seven receiving the same product without iron supplement (iron concentration less than 0.5 mg/l) were examined during the first 12 weeks of life. In breast-fed infants bifidobacteria was predominant, counts of Escherichia coli were low, and other bacteria were rarely present. Infants receiving fortified cow-milk preparation had high counts of Escherichia coli, counts and isolation frequency of bifidobacteria were low and other bacteria were frequently isolated. In those on unfortified cow-milk preparation isolation frequency of Escherichia coli, bifidobacteria and bacteroides was comparable with that in breast-fed infants; however, counts of Escherichia coli were high. It is concluded that the faecal flora of infants fed unfortified cow-milk preparation acquires characteristics of that found in breast-fed infants.