Bilirubin determinations were obtained at weekly intervals from 1 to 6 weeks of age in 27 breast-fed and 12 formula-fed term, average size infants. Mean bilirubin levels were significantly higher in breast-fed infants at each age studied. The highest mean bilirubin level in each group was present at 1 week of age, 10.6 (+/- SD 4.6) mg/dl in breast-fed and 4.7 (+/- SD 3.0) mg/dl in formula-fed infants. Thereafter, values gradually fell in both groups. Mean birthweight was not different between the two groups; however, breast-fed infants lost significantly more weight (4.8%) by the time of discharge than formula-fed infants (2.2%). Breast-fed infants remained significantly lighter in weight at 6 weeks of age. Analysis of variance and covariance revealed no significant correlation between body weight, weight change in grams or percent weight change, and bilirubin levels in either group. These data indicate that mean bilirubin levels are significantly higher in breast-fed infants compared with formula-fed infants from 1 to 6 weeks of age. Breast-fed infants also have a significantly greater weight loss during the first four days of life and remain lighter at 6 weeks of age; however, there is no relationship of weight loss to bilirubin levels.