Concentrations of free amino acids were determined quantitatively by ion exchange column chromatography in serum from the fasting blood of 352 (male = 177, female = 175) "healthy" infants, children and adolescents. Serum was deproteinised by 5% sulphosalicylic acid (1:1, v/v). The results -95% confidence range-are presented age-dependent (groups: up to 1 month, 1-24 months, 2-6 years, 6-10 years, 10-14 years) and sex-dependent in table 4 and 5.