Six clonal lines of human choriocarcinoma have been established from a tumor previously adapted to passage in the hamster cheek pouch. All clones have continued to produce chorionic gonadotropin (CG) for over 2 yr. The rates of hormone secretion and growth differed among the clonal lines. In one clone, cell count and CG production showed log increases and plateau phases, but the log phase of CG synthesis occurred as the rate of cell replication slowed. Accordingly, CG production per cell increased progressively with time. These functional cell lines provide systems for the examination of many aspects of placental physiology and biology.