Among 833 cancer patients whose sera were investigated for Regan isoenzyme and among 1,319 cancer patients from a different population whose sera were assayed for human chorionic gonadotropin (HCG), those patients with neoplasms of the testis or ovary showed the highest frequency of both placental proteins. Among another 22 patients with ovarian cancer, for whom both placental proteins were measured, 59% showed Regan isoenzyme and 68% showed HCG in ascitic fluids, whereas the figures were 65% and 30%, respectively, for sera. In 55% of both fluids and sera, there was a positive correlation of Regan isoenzyme with HCG (positive or negative). Almost invariably, the ascitic fluid was richer in Regan isoenzyme and HCG than the serum when both were collected on the same day. Progressively increasing levels of each placental protein generally correlated with the spread of the disease, though there were instances when only one was expressed. Evidence indicated the existence of two forms of alkaline phosphatase in ovarian cancer, Regan and non-Regan; the latter was assumed to be of fetal origin. Ultrastructural studies of one ovarian cancer revealed a morphologic entity, i.e., mitochondria enveloped by inverted tubules of endoplasmic reticulum.