The Btk (Phospho-Tyr551) Colorimetric Cell-Based ELISA Kit provides a lysate-free, convenient, and high-throughput approach for sensitive detection of Btk (Phospho-Tyr551) protein expression profile in cells. The kit can be used for measuring the relative amounts of Btk (Phospho-Tyr551) in cultured cells as well as screening for the effects that various treatments, inhibitors (ie siRNA or chemicals), or activators have on Btk (Phospho-Tyr551).