The present invention relates to a method for preparing a composition comprising monomeric conjugates from a sample, said conjugate comprising (a) a polypeptide comprising the amino acid sequence of interleukin 15 or derivatives thereof, and (b) a polypeptide comprising the amino acid sequence of the sushi domain of IL-15Rα or derivatives thereof； wherein said method comprises the use of anion-exchange chromatography followed by a hydrophobic interaction chromatography； and to a pharmaceutical composition which can be obtained by such a method.
