A process for the hydrolysis of the amino-acid chain of preproinsulin of the formula I (SEQ. ID NO: 2) ##STR2## in which R1 is n amino acids, where n is the integer 0 or 1,R2 is hydrogen, an amino acid which can be cleaved off chemically or enzymatically, or a peptide with 2 to 30 amino-acid residues, R3 is a hydroxyl group, an amino acid or a peptide with 2 to 10 amino acids, X is L-arginine or a