Abstract: Methods of removing protein contaminants from a solution comprising thrombopoietin are disclosed. The solution is exposed to hydroxyapatite, whereby protein contaminants are bound to the hydroxyapatite and the thrombopoietin remains substantially unbound, and the unbound thrombopoietin is recovered. The use of hydroxyapatite is advantageously combined with other purification and concentration techniques, including ion-exchange chromatography, ligand affinity chromatography, hydrophobic interaction chromatography, ultrafiltration, and differential precipitation.