Steroid sulfatase (STS), an important enzyme in the pathway of estrogen synthesis from sulfated steroid precursors, was localized to the syncytial trophoblast of human placentas during different periods of pregnancy by using a mouse monoclonal antibody and immunocytochemical techniques. Preembedding immunoelectron microscopy revealed STS immunoreactivity associated with the rough endoplasmic reticulum of the syncytial trophoblast. STS mRNA was also localized to this outermost layer of the human trophoblast. At a cellular level, both STS message and immunoreactivity seemed to be more abundant during the late first and early second trimesters than in term placentas. Although the syncytial trophoblast is known to originate from the cytotrophoblast, neither STS immunoreactivity nor STS mRNA was detected in the cytotrophoblast at any stage of placental development studied.