Abstract To explore the possibility that the placenta prevents its own immunologic rejection by the mother by producing a “hormonal barrier” either locally or systemically, rabbits undergoing skin allograft exchanges were given placental hormones either systemically or at graft sites. Groups of 5 animals each received estrogen, progesterone, chorionic gonadotropin, or placental lactogen in addition to appropriate controls including cortisone and crude placental extract. No allograft prolongation was detected using either systemic or local placental hormones. The possible reasons for failure to demonstrate immunologic activity of these hormones are discussed.