The activity of granzyme B, a main effector molecule of natural killer (NK) cells and cytotoxic T lymphocytes, is regulated by the intracellular serine proteinase inhibitor 9 (PI-9). Pig PI-9 was first cloned, and the sequences that encode pig PI-9, including the start codon and stop codon, were identified. The cDNA was inserted into the cloning site of pCXN2 (chicken beta actin promoter and cytomegalovirus enhancer), transfected into pig endothelial cells (PEC), and several stable PEC clones were established. An NK cell-mediated cytolysis test was next applied to the PEC clones, using YT cells (an NK-like cell line). The PEC transfectants with pig PI-9 had a significant inhibitory effect on NK cell-mediated PEC lysis. The overexpression of the anti-apoptotic molecule, pig PI-9, has the potential for use in protecting graft cells from human NK cells.