Serum gamma-glutamyl transpeptidase (GGT) activities of 82 healthy neonates (aged 9 hours to 11 days) and 106 healthy children (aged 2 months to 15 years) were determined. Serum GGT activity of 47 neonates (51%) was higher than the accepted upper limit of normal for adults. By three months of age, all of the children had serum GGT activities that were within the accepted normal range for adults. Thereafter there was only minimal variation in serum GGT activities of older children. Although mean serum GGT activity was higher in male children than in female children, there was no significant difference between the values for male and female neonates. That after the neonatal period serum GGT activity is constant in the adult range and is not affected by bone growth as is alkaline phosphatase suggests that GGT may be of value in the evaluation of hepatobiliary disease in children.