Escherichia coli was found in a similar proportion of stool specimens from infants who were breast-fed and from others fed on three different artificial-milk preparations. When E. coli was present its mean colony count in the stools of breast-fed infants was within the range of the mean counts for infants receiving the artificial -milk feeds. There was no consistent relation between high counts of bifidobacteria (Lactobacillus bifidus) and low counts of E. coli. This suggests that measures aimed at implanting or stimulating the growth of bifidobacteria in the large intestine of artificially fed infants may not greatly influence the E. coli population therein. The results are discussed in relation to the protection of artifically fed infants from E. coli enteritis.