(57) In the method the cut flax or other rettable plant is treated with an SPS-ase preparation producible by means of a strain belonging to the genus Aspergillus, preferably to Aspergillus japonicus or Aspergillus aculeatus, in an aqueous medium until the retting is completed or substantially completed, whereafter, if desired, the retted flax or other rettable plant is decolourized by being exposed to a moderate bleaching procedure only. In this manner a number of technical advantages are obtained, especially a tremendous time saving, a higher yield, a higher strength, a lighter colour and a better reproducibility.