We describe a method for purifying chymosin comprising providing an aqueous liquid sample containing chymosin and a separation medium comprising a base matrix and a plurality of firmly attached ligands that are capable of binding to chymosin, contacting the matrix with the sample under conditions permitting binding of chymosin to the matrix, and desorbing chymosin from the matrix. The characterizing feature is that the matrix is hydrophilic and that the ligands in the plurality of ligands are hydrocarbon groups in which all carbon atoms are sp3-hybridised, possibly with an ether oxygen or a thioether sulphur inserted between two carbon atoms at one or more positions in at least one of the hydrocarbon groups, and possibly a hydroxy group replacing a hydrogen atom at one or more positions in at least one of the hydrocarbon groups.