Our objective was to evaluate whether antiphospholipid antibody-containing sera could play a regulatory role in signal transduction induced by gonadotrophin releasing hormone (GnRH) when incubated with normal human trophoblast cells. To test this hypothesis we established an in vitro placental culture system in which GnRH addition in the presence of normal human serum resulted in a significant increase in human chorionic gonadotrophin (hCG) secretion. When GnRH was added to the culture medium with antiphospholipid antibody-containing sera, the hCG increase was inhibited. These results suggest the possibility that antiphospholipid antibody-positive sera can exert their effect on GnRH-induced signal transduction. Further studies are needed to explain their exact site of action.