Described is a method for the preparation of a mixture of peptides having a cysteine content between 7-20 w/w % from a protein source, comprising cysteine containing proteins, comprising the steps of: a) cleaving the proteins of the protein source into peptides; b) digesting the peptides obtained in step a) by an exopeptidase, the action of which is at least attenuated at the position of a cysteine in the peptide, therewith forming digested peptides having a terminal cysteine; c) purifying the digested peptides, and the use of the preparation as active component in a medicament, especially for the treatment of conditions mediated by oxidative damage and for the elevation of cellular glutathion levels in the human or animal body.