The localization of IL_6 at light microscopic levels and its quantitative analysis in human placentas were studied by using immunohistochemistry. Both syncytiotrophoblast and cytotrophoblast in placental villi, white blood cells in villose capillary cavity stromal cells and capillary endothelium present IL_6 immunoreactivity in cytoplasm. Using a quantitative immunohistochemical method, the amounts of IL_6 in placenta were low at the 6th week of gestation, increased saliently and peaked at the 7th, then reduced gradually during the rest of the gestation period and maintained the same level as that in the 6th week of gestation. This changing trend was paralleled with that of GnRH in our previous studies. These results revealed that IL_6 could be produced by the human placenta and may take part in the regulation of placental hormone releasing. The localization of IL_6 at light microscopic levels and its quantitative analysis in human placentas were studied by using immunohistochemistry. Both syncytiotrophoblast and cytotrophoblast in placental villi, white blood cells in villose capillary cavity stromal cells and capillary endothelium present IL_6 immunoreactivity in cytoplasm. Using a quantitative immunohistochemical method, the amounts of IL_6 in placenta were low at the 6th week of gestation, increased saliently and peaked at the 7th, then reduced gradually during the rest of the gestation period and maintained the same level as that in the 6th week of gestation. This changing trend was paralleled with that of GnRH in our previous studies. These results revealed that IL_6 could be produced by the human placenta and may take part in the regulation of placental hormone releasing.