Growth parameters, biochemical indice of protein metabolism and plasma Amino acid (AA) concentrations were investigated during the first month of life in term infants (n = 61) fed various protein hydrolysate formulas (whey (WHF, n = 3), soy collagen (SCHF, n = 1) and whey-casein hydrolysate formulas (WCHF, n = 1)). In addition, metabolic balance studies were performed in 10 infants fed WHF and in 5 fed WCHF. Comparatively to breast fed infants, growth reduction and decrease in plasma protein concentrations were observed with the use of one of the WHF and in a lesser extent with the SCHF and the WCHF. Plasma amino acid pattern reflected the AA content of the formulas. Whey hydrolysate formulas induced mainly an increase in threonine and a decrease in tyrosine concentrations. Soy-collagen hydrolysate formula led to an increase of non-essential amino acids, such as glycine and hydroxyproline and a decrease in plasma lysine and cystine. Whey-casein hydrolysate formula induced a plasma amino acid pattern close to the profile observed in breast fed infant. Metabolic balance studies showed a relative reduction in nitrogen absorption and utilisation in the infants fed the WHF and the WCHF. In addition a drastic reduction in fat, calcium and phosphorus absorption was also observed with the use of the WCHF. In preterm infants (n = 19) fed whey predominant hydrolysed preterm formulas (n = 3), metabolic balance studies an plasma AA concentration were evaluated at the end of the first month of life at 34 weeks of gestation age. Comparatively to similar preterm infants fed conventional preterm formulas, a relative reduction in nitrogen absorption (83% vs 90%) and retention (64 vs 70%) as well as in phosphorus absorption (78 vs 89%) was observed. Calcium retention was similar (48 vs 45 mg/kg/d) but calcium intake was significantly higher in infants fed hydrolysate formulas 120 vs 91 mg/kg/d. Plasma amino acid concentrations were related to amino acid composition of the formulas. Compared with the standard preterm formulas, all three protein hydrolysate formulas led to a significant increase in plasma threonine and a decrease in tyrosine and phenylalanine concentrations. In addition, there was a reduction in plasma histidine, valine, leucine, cystine, methionine and/or tryptophane with some of the hydrolysate formulas used. In conclusion, these studies provide evidence that protein hydrolysed formulas are not equivalent to whole protein formulas in terms of nutritional efficiency for preterm and term infants. Therefore further extensive nutritional studies on growth, biochemical indices of protein metabolism and metabolic balance, including minerals and trace elements, appear to be necessary before maintaining and promoting the use of such formulas for teh potential benefits on atopic disease in preterm and in full-term newborn infants.