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 SP Sephacryl&trade； S 500 HR chromatography material conditioned to a conductivity of 21 mS/cm and a linear gradient elution a fusion protein of erythropoietin and a single poly(ethylene glycol) residue can be obtained in a single step with high purity and yield.
