Estrogens play an important role in the regulation of placental function, and 17-beta-estradiol (E2) production rises eighty fold during human pregnancy. Although term placenta has been found to specifically bind estrogens, cellular localization of estrogen receptor alpha (ER-alpha) in trophoblast remains unclear. We used western blot analysis and immunohistochemistry with h-151 and ID5 monoclonal antibodies to determine the expression and cellular localization of ER-alpha protein in human placentae and cultured trophoblast cells. Western blot analysis revealed a ~65 kDa ER-alpha band in MCF-7 breast carcinoma cells (positive control). A similar band was detected in five normal term placentae exhibiting strong expression of Thy-1 differentiation protein in the villous core. However, five other term placentae, which exhibited low or no Thy-1 expression (abnormal placentae), exhibited virtually no ER-alpha expression. In normal placentae, nuclear ER-alpha expression was confined to villous cytotrophoblast cells (CT), but syncytiotrophoblast (ST) and extravillous trophoblast cells were unstained. In abnormal placentae no CT expressing ER-alpha were detected. Normal and abnormal placentae also showed ER-alpha expression in villous vascular pericytes and amniotic (but not villous) fibroblasts; no staining was detected in amniotic epithelial cells or decidual cells. All cultured trophoblast cells derived from the same normal and abnormal placentae showed distinct ER-alpha expression in western blots, and the ER-alpha expression was confined to the differentiating CT, but not to the mature ST. Trophoblast cells from six additional placentae were cultured in normal medium with phenol red (a weak estrogen) as above (PhR+), or plated in phenol red-free medium (PhR-) without or with mid-pregnancy levels of E2 (20 nM). Culture in PhR- medium without E2 caused retardation of syncytium formation and PhR-medium with E2 caused acceleration of syncytium formation compared to cultures in PhR+ medium. These data indicate that the considerable increase in estrogen production during pregnancy may play a role, via the ER-alpha, in the stimulation of CT differentiation and promote function in normal placentae. This mechanism, however, may not operate in abnormal placentae, which show a lack of ER-alpha expression.