Natural killer (NK) cells are a major population of lymphocytes in the human endometrium (EM), and NK cells can be a significant source of cytokines that alter local immune responses. The aim of this study was to determine the expression of NK cell receptors in situ and to test whether uterine NK (uNK) cells produce cytokines and how this activity may be regulated by transforming growth factor-beta (TGF-beta). We observed that human uNK cells were CD56+, CD3-, CD57-, CD9+, CD94+, killer inhibitory receptor+, and CD16+/- in situ by confocal microscopy. We examined cytokine production by uNK cells and uNK cell clones derived from human EM. Stimulation of uNK cells with interleukin (IL)-12 and IL-15, both of which are expressed in the human EM, induced interferon-gamma (IFN-gamma) and IL-10 production. IFN-gamma production by uNK cell clones was completely inhibited by TGF-beta1 in a dose-dependent manner with an inhibitory concentration 50% value of 20 pg/ml. IL-10 secretion by uNK cell clones was also inhibited by TGF-beta1 at similar concentrations. Furthermore, blocking endogenous TGF-beta in fresh human endometrial cell cultures increased the production of IFN-gamma by uNK cells. These data indicate that uNK cells have a unique phenotype that is distinct from blood NK cells. Further, data demonstrate that uNK cells can produce immunoregulatory cytokines and that inhibition of uNK cells by locally produced TGF-beta1 is a likely mechanism to regulate NK cell function in the human EM.