Total serum bile acids were determined in 83 infants of different classification. In 33 mature infants total bile acids were measured after feeding human milk (n = 17) or formula (Milasan). Statistically significant elevations of total bile acids were found on the 14th day of life, if mature infants were formula fed. In low-birth-weight infants appropriate for gestational age total bile acids are not higher on the 8th day of life than in mature infants, but on the 14th day of life the elevations become significant. Low-birth-weight infants small for gestational age and infants suffering from fetal distress syndromes show the highest total bile acids with great variations. Low-birth-weight infants small for gestational age had serum total bile acid concentrations related to the degree of fetal growth retardation (r = -0.871). Some newborns suffering from late metabolic acidosis had lowest intraluminal total bile acids, especially dihydroxy bile acids. We found a direct hyperbolic dependence between total bile acids in the serum and the duodenal content (r = +0.93). In cases of serum total bile acids above 50 mumol/l, concentrations in duodenal content do not exceed 2 mmol/l. Therefore, the degree of fetal growth retardation, fetal distress syndromes, and the nutrition after birth have great influence on bile acid metabolism.