Serum total bile acids were measured using an enzyme micromethod involving 3-hydroxysteroid dehydrogenase (Sterognost 3 Fluorometry) without prior extraction. The test specimen was 50 microliters of serum, with reading by spectrofluorometry. The degree of accuracy assessed at several concentration levels was acceptable for values greater than 5 mumol/l. The calibration function was linear between 0 and 80 mumol/l. Lower limit of detection was 0.88 mumol/l. Results given by this method were compared with those obtained using the Schwarz technique involving extraction of bile acids and their enzyme estimation. The correlation between the two methods was satisfactory. Absence of interference by bilirubin was confirmed. Total bile acids were measured in the serum of fasting children, aged from 1 day to 4 years. High values were seen at birth (m = 8.33 mumol/l, SD = 3.46, n = 10) as well as during the first month of life. Serum total bile acid concentration decreased up to the age of 10 months (m = 2.13 mumol/l, SD = 1.91, n = 31). After one year, they increased progressively, reaching at the age of 4 values similar to those in the adult (m = 3.80 mumol/l, SD = 2.17, n = 30). Particularly high values were found in children with delayed growth or in premature infants (m = 19.66 mumol/l, SD = 10.4, n = 15). Serum total bile acid levels were lower in umbilical cord blood and in the newborn. The increase in serum bile acids found in the newborn may be due to slight cholestasis or to a reduction in the hepatic clearance, showing the special type of hepatic function in the young child.