The present invention relates to a method for purifying a cysteine containing recombinantly expressed protein comprising at least 2, preferably 3 or 4 and even more preferably all of the following steps: (a) sulphonation of a lysate from recombinant host cells or lysis of recombinant host cells in the presence of guanidinium chloride followed by a subsequent sulphonation of the cell lysate, (b) treatment with a zwitterionic detergent, preferably after removal of the cell debris, (c) purification of the sulphonated version of the recombinant protein or purification of the sulphonated version of the recombinant protein with subsequent removal of the zwitterionic detergent, with said purification being preferably chromatography, more preferably a Ni-IMAC chromatography with said recombinant protein being a His-tagged recombinant protein, (d) desulphonation of the sulphonated version of the recombinant protein, preferably with a molar excess of DTT, (e) storage in the presence of a molar excess of DTT.
