During normal mouse pregnancy, abundant numbers of uterine natural killer (uNK) cells differentiate at implantation sites and contribute to early, post-implantation endometrial angiogenesis and to spiral arterial modification. Mouse uNK cells are confidently recognized by light microscopy in tissue sections stained with special protocols. Classically, mouse uNK cells were identified as lymphocytes containing Periodic Acid Schiff's (PAS) reactive cytoplasmic granules. More recently, Dolichos biflorus lectin (DBA) reactions which stain not only the cytoplasmic granules but also the uNK cell membranes have been widely adopted. No lymphocytes in any tissues of virgin mice or external to the uterus of pregnant mice have DBA lectin reactivity equivalent to that of uNK cells; however, some uNK cells are now recognized as DBA-. Here, we describe a PAS/DBA lectin double staining protocol and assess the coincident staining of C57Bl/6J uNK cells from gestation day (gd)6, the first day of uNK cell abundance, to gd12, the day when Tunel+ nuclear senecence appears widely in uNK cells before their numerical decline. For these gd, PAS+ DBA- and PAS+DBA+ cells but not PAS-DBA+ cells were identified. Dual positive cells increased from 47% at gd6 to 85% at gd12. Transplantation of normal bone marrow into alymphoid mice who were subsequently mated revealed the uterus repopulated by doubly reactive PAS+DBA+ uNK cells (>95%). Thus, in normal pregnancies, most uNK cells appear to arise from progenitor cells that have homed to the uterus.