The process of conjugation and secretion of bilirubin was studied in a group of healthy, full-term, exclusively breast-fed newborns and a control group of exclusively formula-fed infants by means of a reverse-phase high-performance liquid chromatographic analysis of bilirubins present in serum. The serum concentrations of unconjugated bilirubin, esterified bilirubin, and the proportion of diesterified bilirubin (as percent of esterified bilirubin) were not significantly different in breast- and formula-fed infants on the 3rd and 5th days of life. These data suggest that both bilirubin production and conjugation are not different in breast-fed and in formula-fed newborns.