Membrane-bound human leukocyte antigen-G (HLA-G) molecules are primarily expressed by cytotrophoblasts of the fetus. They are thought to protect the fetus from immunologic attack by the maternal immune system and have recently been associated with transplantation graft acceptance. In addition, soluble HLA-G molecules (sHLA-G) have been shown to play a role in the success of pregnancies, but are upregulated in certain cancers. However, the exact mechanism for this regulation has remained elusive. The aim of this study was to examine the mechanism by which sHLA-G interact with natural killer (NK) cells in vitro. sHLA-G effectively blocked NK lysis of target cells via fracticide killing of NK cells by apoptosis. These studies support the protective role of sHLA-G in immunologic reactions by interacting with NK cells, thus providing a regulatory function.