Uterine Natural Killer (uNK) cells are a transient lymphocyte population found in the pregnant uteri of human and rodents. The pregnant uterine environment appears to influence migration, differentiation and suppression of the cytolytic activation of uNK cells but the mechanisms involved in these processes are not well understood. Similarities to circulating NK (cNK) cells are limited. The present study sought to discrimate uNK cells from cNK cells in mice by identification of a unique uNK cell marker. Dolichos biflorus (DBA) lectin, which has high selectivity for glycoconjugates containing N-acetyl D-galactosomine in the terminal position, reacted with the plasma membranes of mouse uNK cells. DBA lectin did not react with other uterine lymphocytes or with cNK cell surfaces in Swiss, CBA-J, C57BL/6, SJL, BALB/c, DBA-2 mice strains. DBA lectin staining was useful for both light and electron microscopy and distinguished 4 uNK cell subtypes that appear to be stages of differentiation. Quantitative evaluation of these 4 uNK cell subtypes over early to late gestational times showed dynamic changes between immature and mature forms in different compartments of the implantation sites and indicated the occurrence of microdomains in the uterus capable of controlling uNK cell proliferation and differentiation. This is the first report showing mouse uNK cells expressing specific molecules not found in other NK cells. Use of this reagent should enhance studies of earlier, non-granulated forms of uNK cells and provide new strategies for purification of mouse uNK cells for functional and molecular studies.