This invention describes a process for separating a biologically active polypeptide or protein in the form of its precursor from a mixture containing said precursor and impurities, which comprises contacting said precursor with a resin containing immobilized metal ions, said precursor comprising the biologically active polypeptide or protein covalently linked directly or indirectly to an immobilized metal ion chelating peptide, binding said precursor to said resin, and selectively eluting said precursor from said resin. The corresponding precursor compounds also are described.