In this report, we describe the inhibitory effect of two kinds of human interferons, IFN-alpha and IFN-beta, on the growth of human choriocarcinoma cell lines in vitro and in vivo on comparing the other various cell lines derived from gynecologic malignancies. As compared to human endometrial (HEC-1B) or ovarian (HOC-21) adenocarcinoma cells which were highly resistant to the inhibitory effect of IFN-beta on the DNA synthesis at any doses up to 10,000U/ml, two human choriocarcinoma cell lines, BeWo and HCCM-5, and yolk sac tumor cell line YS-K were relatively sensitive. However, the inhibition was not dose-dependent. The growth of BeWo and HCCM-5 cells were also inhibited by either IFN-alpha or IFN-beta at concentration of 1,000U/ml and the inhibition became conspicuous when the treatment was prolonged to 4 days or more. When the hamsters transplanted with BeWo or HCCM-5 cells were treated with 10,000U IFN-beta on every day from the first till the 7th day after inoculation of cells, the tumor weights decreased to about one-half of those of non-treated controls. In contrast, the tumor weights were almost the same as non-treated controls when the animals were given with IFN-beta from the 7th day till the 13th day.