Fibronectin is an extracellular matrix component which has a crucial role in the establishment of a successful pregnancy. In this respectit plays an important function in cell migration. Uterine Natural Killer cells are the dominant lymphocytes found in the pregnant mammalian uterus. Studies have identified four differentiation stages of mouse uNK cells based on Dolichos biflorus agglutinin lectin cytochemistry. These differ-rentiated cells are distributed in preferential regions in the uterus throughout pregnancy. One function of these cells is the release of cytokines such as IFN-γ leading to spiral artery dilation and the maintenance of decidual integrity.Therefore these cells are essential for the normal development of decidua and placenta. When integrin expression on uNK cells was investigated in mouse uterine sections from gestation day 6 to 17it was found thata5, a6 and ß7 integrins were expressed in cells from gestation day 8 to 10 but not after gd12. Lamininwas expressed only on the antimesometrial side. ß7 integrinis an active receptor for VCAM-1 or MAdCAM-1 expressed by endothelial cells and this function promotes extra vasation of uNK cells through blood vessel walls. The absence of laminin closed to uNK cells suggests that these cells are not dependent on laminin and a6 integrin for their establishment within the extracellular matrix. However, fibronectin is present at gestation day 6 to 10 around uNK cells from the myometrium and the endometrial-myometrial junction. No staining for fibronectin was seen in the decidualized and non-decidualized endometrium near the placenta. Our results suggest that fibronectin appears to support uNK migration, proliferation, differentiation and survival in the uterus by binding toa5 integrin. 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