Chorionic gonadotrophin has immunohisto-chemically been localized to trophoblastic tissue. In normal placenta immunoreactivity was found in syncytiotrophoblastic cells. However, in trophoblastic neoplasms, chorioadenoma destruens and choriocarcinoma, the hormone was also found in cytotrophoblastic cells. The tumor cells showing immunoreactivity appeared to have distinct light microscopic features.