The purpose of this investigation was to provide quantitative data on amino acids ingested by human milk-fed infants. Ten term infants fed human milk ad libitum were studied at 2, 4, and 8 weeks of age. Measurements of milk volume and amino acid analyses of milk samples were performed. Both total amino acid and nitrogen intakes (mumole/kg/day) declined by approximately 34% during the study (P less than 0.001) to 0.05), primarily because of a 20% decline in respective concentrations in human milk (P less than 0.001). Consequently, markedly different amino acid intakes were observed at 2 and 8 weeks. Quantities of amino acids consumed by nearly every infant exceeded estimated Food and Agriculture Organization/World Health Organization requirements at 2 weeks. However, mean intakes for seven of nine essential amino acids studied (phenylalanine, tyrosine, methionine, threonine, valine, leucine, and lysine) were less than recommended quantities at 8 weeks. These same relationships between observed and recommended intakes were also evident at 4 weeks, except for lysine and leucine. Mean intakes of histidine and isoleucine exceeded estimated requirements at all times. These data indicate that FAO/WHO estimates of amino acid requirements may be inflated for phenylalanine, tyrosine, methionine, threonine, valine, leucine, and lysine.