We have shown that oestrogen has a central integrative role in regulating key components of the progesterone biosynthetic and corticosteroid metabolic pathways within syncytiotrophoblasts that govern placental function and maturation of the fetal pituitary-adrenocortical axis. Studies utilizing classic binding procedures and RNAse protection have demonstrated that human placental villous tissue exhibits specific high affinity oestrogen binding and expresses the mRNA for the oestrogen receptor. However, it is not known whether the oestrogen receptor is expressed specifically in syncytiotrophoblasts. Therefore, the present study determined whether the oestrogen receptor protein was detectable by immunocytochemistry in cultured human syncytiotrophoblast maintained in a low oestrogen/progestin environment. Cytotrophoblasts were isolated from human term placentae by trypsin dispersion and Percoll gradient centrifugation and cultured for 5, 7 or 10 days. Incubation of syncytiotrophoblast with 5-10 micrograms/ml of the anti-oestrogen receptor rat monoclonal antibody D-75, which is specific for the primate oestrogen receptor, resulted in identification of the oestrogen receptor in the nuclei of these cells. In contrast, there was no reactivity of the trophoblasts to either rat IgG or an irrelevant rat monoclonal antibody IgG2a against mouse common leukocyte antigen T200. Collectively, these findings indicate that oestrogen receptor is expressed in the nuclei of human placental syncytiotrophoblasts and support the suggestion that the syncytiotrophoblast is an oestrogen-responsive tissue.