Uterine natural killer (uNK) cells expand rapidly during endometrial decidualization and account for 70% of leukocytes in early gestational uteri of humans and rodents. These cells make unique contributions to pregnancy, contributing to the success of embryo implantation and maintenance of decidual tissue that supports placental and fetal development. We postulated that uNK cells express molecules that are not shared by circulating NK (cNK) cells or other leukocytes and, therefore, would be immunogenic for male mice. We isolated viable uNK cells from gestation day 9 pregnant mice and inoculated them into syngeneic males. This induced antibodies reactive with mouse uNK cells but not with cNK cells or other lymphocytes. The antibodies reacted identically with uNK cells in tissue sections from five different mice strains from gestational day 7-12 and in pregnant rat uterus, suggesting that the recognized antigen should be a specific marker of uNK cell. Spleen cells from inoculated males were used subsequently to produce a monoclonal antibody reactive to a uNK cell surface antigen. These experiments confirm that uNK cells are a pregnancy-specific subset of NK cells expressing distinct surface antigen from those found in other tissues.