The response of plasma amino acids to two bovine protein formulas with different protein content (1.6 and 1.2 g/100 ml containing 60% whey proteins and 40% caseins) was measured in term infants. These two groups of infants were compared with a group of infants that were breast-fed; all infants were fed ad libitum. Concentrations of threonine, valine and total branched chain amino acids reflected the amount of protein provided. Thus, the concentrations were higher in the higher protein formula infants from the second week of the study. In the low protein formula infants these amino acids were lower but differed from the infants on breast milk at eight and twelve weeks. Concentration of taurine was lower in the formula fed infants than they were in breast-fed infants at the end of the study. The valine/glycine ratio in the low protein formula group was lower than in the breast-fed group for the first four weeks of the study. After this time it was equal to that of the breast-fed group. These differences in plasma amino acid concentrations give further evidence that formulas now in common use for term infants provide a protein intake in excess of protein requirements after the first months of life.