A biological evaluation of the nutritive value of a whey protein concentrate, as well as of a specific whey protein fraction low in phenylalanine and tyrosine, has been performed by means of an NPU assay. The nutritive values were found to be high, higher, in fact, than what could be expected from the theoretical calculations based on chemical analyses and calculation of chemical scores. The use of whey proteins, low in phenylalanine, in dietary treatment of phenylketonuria is proposed.