SummaryThe effect of feeding hydrolysate infant formula (HF) on protein and amino acid metabolism was investigated in healthy, full-term infants who were either breast-fed (BF, n = 10) or received conventional formula (CF, n = 10) or HF based on soy and beef collagen (n = 10) with equal total protein equivalent contents. There were no differences between groups for gain in weight, length, and head and chest circumferences throughout the study. Plasma concentrations of total proteins, albumin, urea nitrogen, uric acid, and creatinine as well as total amino acid and total essential amino acid concentrations did not differ at the ages of 2, 4, and 8 weeks. In contrast, significant differences were seen in concentrations of five free amino acids. Arginine concentrations were significantly higher at the age of 4 weeks in the infants fed HF than in the other groups (71 ± 12 versus 27 ± 6 and 30 ± 4, μmol/L, mean ± SEM, HF versus BF and CF, p < 0.01). Plasma histidine concentrations were also higher in infants receiving HF (4 weeks: 217 ± 33 versus 91 ± 18, HF versus BF, p < 0.01; 8 weeks: 218 ± 33 versus 105 ± 20, HF versus CF, p < 0.01). The most pronounced feeding-related differences were seen in plasma glycine concentrations (2 weeks: 653 ± 89 versus 345 ± 55, HF versus BF, p < 0.01; 8 weeks: 613 ± 74 versus 385 ± 56 and 312 ± 46, HF versus BF and CF, p < 0.01), with the mean value exceeding the upper limit of the normal range in infants fed HF. Although the biological importance of the differences observed remains to be clarified, further investigations on amino acid metabolism are needed to establish the final nutritional safety of feeding hydrolysate infant formulae.