Breast milk and formula milk and the corresponding serum samples from 20 breast-fed babies, 20 formula-fed babies, and their mothers were examined at 3 days of age for beta-glucuronidase enzyme. Serum indirect bilirubin levels were also examined for all the infants. Serum indirect bilirubin concentrations were significantly higher (p < 0.001) in breast-fed (4.87 +/- 2.4 mg/dl) than in formula-fed infants (1.04 +/- 0.5 mg/dl). beta-glucuronidase activity in formula milk was negligible, while that in human milk was considerable (468.26 +/- 220.8 Sigma units/ml) and was correlated (p < 0.05) with that in the serum of the breast-fed (66.13 +/- 18.1 Sigma units/ml) than in formula fed infants (52.08 +/- 11.9 Sigma units/ml) and a significant (p < 0.05) correlation was found between its serum level and serum indirect bilirubin in both breast and formula fed infants. Also in breast-fed infants serum bilirubin concentrations were related to beta-glucuronidase activity in breast milk (p < 0.05): Breast milk beta-glucuronidase--by facilitating intestinal reabsorption of bilirubin--seems to be an important factor in the neonatal hyperbilirubinemia of breast-fed babies.