Abstract The features of trophoblastic tissue derived from the in vitro culture of marmoset monkey embryos have been described. Long-term trophoblast cultures (in excess of three years in one case) were established from the primary trophoblast monolayer of four of 38 embryos; division of one of these embryos produced two long-term cultures. The trophoblast cells retained their ability to synthesize and secrete chorionic gonadotrophin (CG) during maintenance in vitro and were capable of prolonging the luteal phase when transferred to the uterus of marmosets. A characteristic feature of the cultures was the formation of multiple fluid-filled vesicles enclosed by a single layer of cytotrophoblast cells and attached to the culture dish by a small monolayer of syncytiotrophoblast cells. The tissue was propagated by cutting vesicles into small pieces and placing into a fresh culture dish; attempts to subculture using single-cell suspensions were unsuccessful. These cultures provide a convenient source of marmoset CG for purification as well as an in vitro system for studying other secretory products of primate trophoblast.