Protein A affinity chromatography is widely used for antibody purification. However. the leakage of Protein A. though covalently conjugated to chromatography supports. posts a big problem for antibody based pharmaceutical industry where false results could be generated in immunological assays from contaminated Protein A. Protein A ELISA Kit is developed for accurate quantification of the leakage of native Protein A. recombinant Protein A variants and MabSelect SuRe in the presence or absence of antibodies. The kit offers pharmaceutical users a sensitive assay to measure mini amount of Protein A in therapeutic antibody products. It also facilitates Protein A resin manufacturers in monitoring leaching characteristics of their resins under specific conditions.