A gastric choriocarcinoma cell line capable of producing human chorionic gonadotropin (HCG) was studied by sc transplantation and serial passages into nude mice fed under specific pathogen-free (SPF) conditions. The tumor-take rate at the serial heterotransplantation in SPF mice was high (greater than 95%), in contrast with a low rate in coventional animals (35%). The restoration of morphology and function of the original neoplasm were exclusively verified in SPF mice. Multiple lung metastases were found in 2 animals. The production, storage, and excretion of HCG by tumor cells were confirmed by its high content in serum and cystic fluid in tumors and by its intracellular localization. The tumor cells also contained a specific placental alkaline phosphatase in their membranes. The cells were various trophoblastic types ranging from primitive cytotrophoblasts to typical syncytial cells. The hormone effects of the tumor on sex organs of tumor-bearing animals were evident.