A monoclonal antibody is available for the detection of human intracellular serpin, PI-9 (SERPINB9), which is present in cytotoxic and dendritic cells to protect against the pro-apoptotic protease, granzyme B. PI-9 may be unregulated by tumors to evade the immune system. The anti-PI-9 mAb is a mouse monoclonal for use in immunohistochemistry, immunoblotting and FACS applications.