Trichosanthin and alpha-momorcharin are abortifacient proteins extracted from Chinese medicinal herbs. Study of their in vitro cytotoxicities showed that the two proteins selectively injured choriocarcinoma and melanoma cells. Hepatoma cells represented the most resistant cell line among the various cell lines investigated. Cytotoxicity profiles of trichosanthin and alpha-momorcharin differed from those of anti-cancer drugs which interfere with DNA metabolism such as cisplatin, methotrexate and 5-fluorouracil. Radioactive precursor incorporation studies suggested that the two abortifacient proteins inhibited cellular protein synthesis. The marked decrease in secretion of human chorionic gonadotrophin and progesterone by choriocarcinoma cells after treatment with the proteins could be attributed mainly to loss of cells.