A process for covalently binding a tagged protein to a polymer particle comprising: contacting a tagged protein with a chelating agent-polymer particle conjugate wherein said tag comprises at least two histidine residues and at least two lysine residues and said chelating agent is tridentate, tetradentate or pentadentate and comprises at least two carboxyl groups and is coordinated by Co.sup.2+ ions, to form particle-chelating agent Co.sup.2+ complex: contacting said complex with a carbodiimide; and optionally removing the Co.sup.2+ ions.