OBJECTIVE To examine the cellular localization of each inhibin subunit in human placenta throughout pregnancy. METHODS Placental tissues were collected and fixed in Bouin's solution, and studied with the immunohistochemical technique avidin-biotin-peroxidase complex. RESULTS There was immunohistochemical staining with antisera against each inhibin subunit in the syncytiotrophoblast, but not in the cytotrophoblast. In the first-trimester placenta, positive immunostaining for alpha- and beta A-subunits was clearly observed in the syncytial layer of villi, whereas staining for beta B-subunit was faint. In the second-trimester placenta, the relative intensities of staining for alpha- and beta A-subunits were similar to those in the first-trimester placenta, and enhanced positive immunostaining with beta B-subunit antiserum was observed. The relative amount of immunostainable alpha-subunit declined within the syncytiotrophoblast of the third-trimester placenta, whereas levels of immunostainable beta-subunits were unchanged. CONCLUSIONS Inhibin subunits may be produced in the syncytiotrophoblast throughout pregnancy, and activin as well as inhibin may be synthesized in the syncytiotrophoblast of the term placenta.