The CaptureSelect™ FSH Ligand Leakage ELISA detects affinity ligand contamination of FSH purified using CaptureSelect™ FSH Affinity Matrix. The ligand, a 14 kD recombinant protein, is coupled by multipoint attachment through stable amide bonds 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 sensitive ELISA detects affinity ligand contamination of less than 1 ng/mL. The assay is designed to minimize interference and to provide accurate quantitation even in the presence of the protein of interest. The CaptureSelect™ FSH 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.