A desired protein having the formula: A--B--C--P wherein a) A is Lys or Arg, and B and C are arbitrary amino acids, or b) A is an arbitrary amino acid different from Pro, Lys and Arg, and B and/or C is Pro, is produced from a biosynthetically formed amino acid extended protein having the formula: X--A--B--C--P wherein A, B, C and P are as defined above, and X is an amino acid sequence with an even number of amino acids, of which the first one, seen from the N-terminal end, is different from Lys and Arg, all other uneven amino acids are different from Pro, Lys and Arg, and all even amino acids are different from Pro, by reaction with the enzyme dipeptidyl aminopeptidase (DAP I). The desired protein is obtained in a pure state. Thus, e.g. hGH without content of Met-hGH may be produced by the process.