The currently available, commercially prepared amino acid mixtures for intravenous usage do not result in completely normal plasma amino acid patterns. Taking into consideration the known imbalances that occur with the commercial preparations, we designed a new amino acid solution (AF). This solution was prepared to contain a low concentration of those amino acids usually found in increased concentrations in plasma during the course of total parenteral nutrition (TPN) with a commercially prepared product (CF) and a high concentration of branched-chain amino acids and arginine. Plasma amino acid concentrations were determined for nine infants receiving CF (Group A) and eight infants receiving AF (Group B) as the protein source while on TPN for 5 days. These data were compared to 2-h postprandial values obtained from breast-fed, growing infants serving as controls. In comparison with controls, the plasma concentrations of threonine, glycine, methionine, phenylalanine, tryptophan, and lysine were higher in infants in group A, while those of taurine, alanine, valine, cystine, isoleucine, and proline were lower. In group B, the levels of valine and leucine were slightly higher, while those of alanine, cystine, tyrosine, histidine, and proline were lower. Mean nitrogen balances in groups A and B were + 112 and + 170 mg/kg/day, respectively. Therefore, our new amino acid solution may be better for infants receiving TPN.