We have compared the ratio of biologic activity to immunologic activity (B/I) of chorionic gonadotropins produced by 3 clonal lines of choriocarcinoma in culture with the B/I of preparations of human chorionic gonadotropins extracted from the urine of pregnant women. Biologic activity in unextracted culture medium was measured by hypophysectomized immature rat ventral prostate weight and immature mouse uterine weight graded response assays and immunologic activity by radioimmunoassay. The B/I of a commercial preparation of chorionic gonadotropin did not differ from that of the reference preparation, Second International Standard Human Chorionic Gonadotropin, which had a B/I ratio designated 1.00 for the purposes of comparison. Two clonal lines produced chorionic gonadotropin with B/I of approximately 2, one clonal line pioduced chorionic gonadotropin with a B/I of 0.55. Biologic activity determined by 2 assay methods did not differ. The biologic activities of chorionic gonadotropin produced in culture and reference preparation decreased similarly when incubated with anti-human chorionic gonadotropin antibody. There was no significant change in the B/I or biologic activity when the reference preparation was subjected to incubation with choriocarcinoma cells for 24 hr.