The unconjugated and conjugated bile acid levels in sera of 98 normal children and nine normal adults were measured by high performance liquid chromatography. The results showed that the mean total bile acid level was high, 11.0 +/- 8.7 mumol/liter (1 SD) during the neonatal period (0-4 wk) and then gradually decreased with age. The ratio of the concentration of conjugated bile acids to total bile acids in serum was as high as 90% or more in infants under 1 yr of age and slowly decreased with age. The mean ratio of cholic acid to chenodeoxycholic acid was high (1.7 +/- 1.1) during the neonatal period but decreased after 3 months to the adult level (0.4 +/- 0.2). The mean ratio of glycine conjugated bile acids to taurine conjugated bile acids was 3.0 +/- 3.1 during the neonatal period and the ratio during the 1st month of life was significantly lower than that after that period with little further change at any age. The mean ratio of the concentration of secondary bile acids to primary bile acids showed significantly lower values in infants less than 1 yr of age. The main bile acid was glycocholic acid in the neonatal period but after 1-3 months glycochenodeoxycholic acid predominated. With age, the serum bile acid pattern which was characteristic in infancy gradually approached that of adults.