Uterine natural killer (uNK) cells accumulate at the maternal-fetal interface during gestation and are thought to have an important role during pregnancy in both mice and humans. While the cell surface phenotype of human uNK cells is increasingly well defined, less is known regarding the cell surface expression profile of murine uNK cells both before and during gestation. Herein, we demonstrate that murine NK1.1(+) (KLRB1C) endometrial NK (eNK) cells, derived from virgin mice, and NK1.1(+) decidual NK (dNK) cells, obtained from pregnant mice, belong to the B220(+) (PTPRC) CD11c(+) (ITGAX) subset of NK cells. While B220 expression was low on NK1.1(+) eNK cells, it was increased on a subset of NK1.1(+) dNK cells at Embryonic Day 10.5. Endometrial NK and dNK cells also differed somewhat in their expression patterns of two activation markers, namely, CD69 and inducible costimulator (ICOS). The eNK cells acquired a B220(hi)ICOS(+) dNK cell surface phenotype when cultured in vitro in the presence of uterine cells and murine interleukin 15. Thus, the cell surface profiles generated for both NK1.1(+) eNK cells and dNK cells demonstrate that they belong to the recently described B220(+)CD11c(+) subset of NK cells, which are potent cytokine producers.