The ratio of valine/glycine was measured in the umbilical cord blood of 213 newborn infants of singleton pregnancies. The results obtained in small-for-date and preterm infants are compared with those in normal-term infants. A parallel assessment was made of the valine/glycine ratio in venous blood of the mothers of all these infants. An abnormally high rate was found in a small number of infants and also in a few mothers. There did not appear to be any association between these abnormal results and intra-uterine growth retardation.