Human yolk sac carcinomas have been studied only twice with the electron microscope, and have never been compared at this level with normal human yolk sac. In the present study, the ultrastructural features of three primary ovarian yolk sac carcinomas, omental metastases from one of these, and a primary retroperitoneal yolk sac carcinoma in a male are reported, as are the ultrastructural findings in human yolk sac from normal 7- and 12-week gestations. The most prominent feature of the tumors is the presence of voluminous basement membrane material (the nature of which is confirmed by indirect enzyme-labeled antibody technique in one case) in both intra-and extracellular location, corresponding to the PAS-positive hyaline globules seen in these tumors by light microscopy. The tumor cells are also demonstrated to produce this material in tussue culture. Although basement membrane has not been described previously in normal human yolk sac at 8 and 10 weeks' gestation, it was present in the 7-week specimen which we studied, suggesting that its production may be a feature of only very young sac. Other ultrastructural findings are also similar in human yolk sac carcinoma, normal human yolk sac, and rodent yolk sac and yolk sac carcinomas. Thus, these studies confirm the suggested germ cell-derived yolk sac origin of the human tumor.