The present invention pertains to the use of the protein phenylalanine ammonia-lyase, as well as the biologically-active derivatives of the said protein for preventing or treating diseases associated with a phenylalanine imbalance in a human or animal body. More particularly, the present invention relates to the therapeutic use of the above-cited molecules for preventing or treating a phenylalanine imbalance in vivo. This invention also deals with therapeutic compositions comprising a pharmaceutically active amount of the above-described therapeutic molecules as well as with therapeutic methods using the said therapeutic compositions. Finally, the present invention relates to processes for selecting more therapeutically-effective variants of said protein as well as to the selected variants themselves.