Breast milk jaundice (BMJ) is the most common etiology of prolonged unconjugated hyperbilirubinemia in a newborn infant. The pathogenesis of BMJ has been studied by many investigators, yet the etiology still remains uncertain. A prospective study was done to examine the role of breast milk beta-glucuronidase in BMJ in healthy full-term Chinese newborns. There were 71 infants in the formula-fed group (FF group), 28 in the breast-fed group (BF group) and 27 in the mixed feeding group (MF group). Study infants were followed at the well-baby clinic for at least two months. Enzyme activity in maternal breast milk and infants' stools decreased significantly during the first week postpartum. There was no intergroup difference in fecal enzyme activity for the different age groups. Significantly more infants in the BF group (35.7%) developed prolonged unconjugated hyperbilirubinemia than did infants in the FF group (2.8%) and the MF group (0%). We did not find higher enzyme activity in breast milk or stools of infants with BMJ than in infants without BMJ. We, therefore, conclude that breast milk beta-glucuronidase is not related to the development of BMJ.