The relation of age and sex to serum primary (PBA) and total bile acid (TBA) concentrations was evaluated by an enzymatic fluorometric microassay in healthy infants, children and adults. TBA concentrations were the highest in the 6-day-old group and 1-month-old group and seemed to switch to almost normal adult levels by the age of 4-6 years, which persisted throughout life, while PBA concentrations were predominant over a period of 3 days to 3 months after birth. No sex-related differences were observed from neonates to very old persons for any of serum bile acids. These results show that the bile acid metabolism in the liver and the enterohepatic circulation of bile acids are usually matured in infancy and that aging and sex may insignificantly affect serum bile acid metabolic profiles.