The CaptureSelect FcXP Ligand Leakage ELISA is designed to detect possible leached ligand of antibodies or antibody fragments purified using CaptureSelect FcXP Affinity Matrix. The FcXP affinity ligand is a 13-kD recombinant protein, expressed in yeast and covalently coupled to a chromatography matrix for high chemical stability and low leakage. During the chromatography process 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 FcXP 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.