Herein is reported a method for the purification of a protein comprising erythropoietin and a single poly (ethylene glycol) residue from reaction by-products or not reacted starting material by a cation exchange chromatography method. It has been found that by employing a cation exchange Toyopearl® SP-650 chromatography material and employing a second wash step with an increased pH value compared to the first wash step a fusion protein of erythropoietin and a single poly (ethylene glycol) residue can be obtained in a single step with high purity and yield and suitability for large scale applications.