Photometric determination of alpha-amino nitrogen in peripheral venous plasma and urine from 20 healthy, full-term infants, 1--5 months of age, showing normal growth and development during an uncomplicated lactation, revealed lower plasma levels than what has been found in adults, or 3.7 +/- 1.1 mg/100 ml, and a urinary excretion of 41 +/- 14 mg/24 hours. Ion-exchange chromatography of deproteinized peripheral venous plasma showed low valine concentrations, an increased glycine/valine ratio and high cystine and very high taurine levels when compared to the levels of healthy American infants of comparable ages fed 3--3.5 g/kg of cow-milk protein. The findings indicate that a formula based on cow-milk protein should optimally contain only 1.0--1.2 g protein/100 ml provided that it is "humanized" not only with regard to the lactalbumin/casein ratio, but also to the cystine and taurine content. The pattern of the plasma concentrations of free amino acids reported in the present investigation may be used as a normal reference for breast-fed infants.