At the time the human placenta is established, the uterine mucosal lining (decidua) is infiltrated by abundant CD3- CD56bright natural killer (NK) cells. NK cells circulating in blood are known to contain perforin and granzyme A in their cytoplasmic granules. TIA-1, an RNA-binding protein capable of inducing DNA fragmentation, has also been found in the granules of cytolytic cells. In this paper, we demonstrate the presence of perforin, granzyme A and TIA-1 in the granules of uterine NK cells. Sixteen sections of non-pregnant endometrium throughout the menstrual cycle and six sections of early decidua, together with cytospins of four preparations of isolated decidual leukocytes were stained by both immunohistology and immuno-electron microscopy to localize perforin, granzyme A and TIA-1 to the cytoplasmic granules of CD56+ cells. The presence in vivo of these cytolytic molecules in a normal physiological situation implies that these uterine NK cells may have effector functions in the control of normal placentation.