A europium chelate is covalently conjugated on lysines of the anti-GST mouse monoclonal antibody (clone GST.B6). The molar ratio between terbium and antibody in different lots is ~5 - 9. The europium-anti-GST mouse monoclonal antibody recognizes GST fusion proteins and can serve as the donor in TR-FRET assays that offer a sensitive tool in monitoring molecular proximity, post-translational modifications, enzyme activity, and protein abundance. The acceptor is far-red fluorescent dyes, such as Cy5.