The CaptureSelect CH1-XL Ligand Leakage ELISA Kit is designed to detect possible leached ligand of antibodies or antibody fragments purified using CaptureSelect CH1-XL Affinity Matrix. The ligand, a 14-kD recombinant protein, is covalently coupled for 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 CH1-XL Ligand Leakage ELISA Kit can be used as a tool to aid in development of an optimal purification process and in routine quality control of in-process streams and final product.