SYNOPSISUrinary and serum amino‐acid patterns were studied in 56 premature and full‐term babies by a new technique of combined electrophoresis and chromatography. A changing pattern of urinary amino‐acid excretion with increasing maturity was found, particularly with the basic amino acid lysine and with cystine. We have confirmed previous reports of high excretion of proline and hydroxyproline in infants. High levels of tyrosine in the blood and urine in 4 of the 37 premature babies were possibly due to a combination of high protein intake and late maturation of the enzyme tyrosinase in these infants.The excretion of glutamine was surprisingly low. Possible explanations for this are discussed.Homocitrulline was found in blood and urine of all infants fed with sterilized milk, including breast milk. This is thought to be related to the heating of the milk.Apart from tyrosine and homocitrulline, the pattern of serum amino acids did not differ significantly from that seen in normal adults.Infants malnourished in utero showed amino‐acid patterns which resembled those of babies of similar birthweight rather than similar m