The urokinase plasminogen activator (uPA) system plays pivotal roles in cell invasion, adhesion and migration. Roles for uterine natural killer (uNK) cells in regulating extravillous trophoblast (EVT) invasion and spiral artery remodeling have been proposed. Placental bed biopsies from early pregnancy were obtained from three gestational age groups (8-10, 12-14 and 15-20 weeks). Total caseinase activity in the placental bed was studied using casein in situ zymography. Localisation of uPA, uPA receptor (uPAR), plasminogen activator inhibitor (PAI)-1 and -2 in the placental bed was investigated by immunohistochemistry. CD56(+) uNK cells were separated from collagenase-digested decidual cells using an immunomagnetic technique, and uPA activity was measured in isolated cell culture supernatants by casein/plasminogen gel zymography (8-10 and 12-14 weeks' gestation, n=10 each group). uPAR in cell lysates and PAI-1 and -2 secretion in supernatants were measured by Western blotting. Caseinase activity was stronger in decidua than myometrium as shown by in situ zymography. uPA localised strongly to uNK cells, especially at 8-10 weeks. Moderate uPAR localisation on uNK cells also observed. There was very weak immunostaining of uNK cells for PAI-1 and PAI-2. In casein gel zymography, uPA activity was similar in uNK cell culture supernatant compared with total unseparated decidual cells. uPAR in uNK cell lysates was significantly stronger than in total decidual cell lysates. PAI-1 and PAI-2 were not detected in uNK cell culture supernatants by Western blot analysis. These results suggest that uNK cells may regulate EVT invasion and spiral artery remodeling via the uPA system.