Interferon-gamma (IFN-gamma) is a lymphokine, produced by activated T lymphocytes, which plays a key regulatory role in the host immunological responses. In addition, IFN-gamma is expressed by human and porcine trophoblast. As IFN-gamma exerts its biological functions through specific cell surface receptors and a great number of IFN-gamma receptors (IFN-gamma R) have been purified from human placenta, we have examined the relative distribution of IFN-gamma and IFN-gamma R in human placental tissues at different stages of pregnancy. By using immunohistochemical analysis and monoclonal antibodies, it was found that IFN-gamma expression is intense in the first trimester but almost imperceptible at term, whereas the expression of IFN-gamma R is present at both stages of pregnancy. For both lymphokine and receptor, the most intense expression was observed in villous syncytiotrophoblast and in extravillous interstitial trophoblast. From these results it appears that the expression of IFN-gamma R in trophoblast is related to the presence of the lymphokine in the early phase of gestation but not later. On this basis, it can be argued that IFN-gamma exerts its functional role via an autocrine and/or a paracrine loop mainly during the first trimester.