The CaptureSelect C-tagXL Ligand Leakage ELISA is designed to detect possible leached ligand of C-terminal E-P-E-A tagged target proteins purified using CaptureSelect C-tagXL Affinity Matrix. The ligand, a 13 kD recombinant protein, is covalently coupled to give high chemical stability and low leakage. Even when covalently attached, small amounts of the affinity ligand can leach off of the chromatography support and co-elute with the protein. This ELISA provides typical sensitivity and accurate quantitation of possible leached ligand greater than or equal to 1 ng/mL. The CaptureSelect C-tagXL Ligand Leakage ELISA can be used as a tool to aid in optimal purification process development and in routine quality control of in-process streams as well as final product.