The aim of the study was to compare, during the first month of life, growth parameters, biochemical indices of protein metabolism and plasma amino acid concentrations in newborn infants fed either human milk (n = 23), three different whey hydrolysate formulae (WHF 1, n = 13; WHF 2, n = 10; WHF 3, n = 13), a soy-collagen hydrolysate formula (SCHF n = 18) or a whey-casein hydrolysate formula (WCHF, n = 20). Growth parameters and the various protein concentrations determined in the infants fed WHF 1 and WHF 2 were similar to the values observed with human milk. With WHF 3, growth in weight, length and head circumference and serum total protein concentrations were reduced significantly whereas blood urea nitrogen was increased. With SCHF, growth in weight and length as well as serum total protein and transferrin concentration were decreased significantly, whereas serum IgG concentration was increased. With WCHF growth in length and serum transferrin concentration were decreased compared to the human milk group. In the various groups, the plasma amino acid pattern reflected the amino acid content of the formula. Whey hydrolysate formula 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 with human milk. Nevertheless, the plasma concentrations of most of the various amino acids were higher.(ABSTRACT TRUNCATED AT 250 WORDS)