Breast milk beta-glucuronidase was thought to be one of the etiological factors in the pathogenesis of late-onset breast-milk jaundice, but results of these studies are conflicting. In this study breast milk beta-glucuronidase levels were determined in groups with physiologic jaundice, early breast-feeding jaundice and late breast-milk jaundice. No difference in beta-glucuronidase levels of these three groups was found in samples taken on the 4th and 15th days of life. beta-glucuronidase activity in breast milk declined from the 4th to 15th day in all groups. These results imply that factors other than breast milk beta-glucuronidase activity should be investigated to reveal the pathogenesis of late-onset breast-milk jaundice.