Breast milk samples from mothers of breast-fed, healthy, term newborns with unexplained prolonged jaundice were analyzed for beta-glucuronidase activity. Mean enzyme activity was 75.7 +/- 34.5 modified Sigma units/ml in the breast milk samples ingested by the study group of jaundiced babies (n = 25) and 82.2 +/- 40.1 modified Sigma units/ml in the samples ingested by the control group of non-jaundiced babies (n = 20) (p > 0.05). Enzyme activities at 2, 3 and 4 postnatal weeks were 101.0 +/- 39.9, 66.0 +/- 20.7 and 57.0 +/- 22.4 modified Sigma units/ml in the study group and 87.9 +/- 36.1, 58.5 +/- 15.0 and 88.3 +/- 49.1 modified Sigma units/ml in the controls. The differences were not statistically significant (p > 0.05). We conclude that breast milk beta-glucuronidase activity may be a contributory factor, in the presence of other variables, in hyperbilirubinemia but it is neither the main nor the only cause of prolonged jaundice in neonates.