Abstract: A process for the purification of proteins from solutions containing contaminants of similar net charge and molecular weight is provided, comprising contacting a solution containing the desired protein with an immobilized metal affinity chromatography resin in a buffer containing a low concentration of a weak ligand for the chelant of the resin. The adsorbed protein is then eluted using a buffer having a high concentration of the same weak ligand, e.g., Tris. Particularly preferred features employ agarose-iminodiacetic acid resins having copper cations and are especially useful in obtaining preparations of homogeneous, stable rsT4 proteins.