Eight independent cell lines, derived from human testicular germ-cell tumors, were examined for the expression of various markers. These included major histocompatibility and embryonic antigens, chorionic gonadotropin, alpha fetoprotein, alkaline phosphatase, plasminogen activator, and infectivity by SV40. No line consisted primarily of choriocarcinoma or yolk sac cells, but several contained cells resembling murine embryonal carcinoma; some of these lines formed tumors with the distinctive features of embryonal carcinoma when injected into immunosuppressed animals. It is proposed that human embryonal carcinoma cells, unlike those of the mouse, correspond to a preblastocyst stage of development.