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Natural killer (NK) cells are capable of lysing cells with missing or altered surface expression of MHC class I molecules. MHC class I molecules provide inhibitory signals to NK cells through the interaction with NK inhibitory receptors. There is increasing evidence that the NK cell mediated cellular response plays a role in xenograft rejection. In this study, fresh PBL which comprises polyclonal NK cell population was isolated from seven healthy donors carrying either homozygous HLA - Cw3 or -Cw4 alleles to be used as NK effector cells. Xenogeneic porcine endothelial cells (PAEC!) but not human allogeneic target cells were susceptible to polyclonal human NK cell mediated lysis. Fresh isolated PBL was able to lyse 20-30% of PAEC, as compared with less than 5% of allogeneic human target cells. As expected, these NK cells also lysed NK susceptible human erythroleukemic cell line, K562. This demonstrates porcine MHC class I molecules are unable to provide inhibitory signals to human NK cells. In order to determine whether porcine target cells can be protected by HLA-G, a non-classical human MHC class I molecule which can provide inhibitory signals to both group I and group II NK subpopulation, human full length genomic HLA-G DNA was transfected into PAEC by lipofectamine transfection. The expression of HLA-G was confirmed by FACS analysis after staining with a monoclonal antibody against HLA class I framework determinants, PA2.6. Expression of HLA-G lead to a significant blocking of NK mediated cytolysis (range 52 to 100% inhibition) in all the individual tested. Thus, our results, for the first time, demonstrate that expression of HLA-G on xenogeneic cells, such as PAEC, alone is sufficient to protect otherwise susceptible xenogeneic target cells from polyclonal human NK cell lysis