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The capacity for differentiation of five human embryonal carcinoma (EC) cell lines was studied by examination of the morphology of xenograft tumors in nude mice. In addition to morphology, differentiation into extraem-bryonic elements was studied by examining the evidence of human chorionic gonadotropin (hCG) and α-fetoprotein (AFP) production. The EC cell lines can be divided into two groups: pluripotent (NEC 14 and Tera 2) and nullipotent (NEC 8, NEC 15, and ITO-II) cell lines. In nude mice, the pluripotent NEC 14 and Tera 2 cells produced EC mixed with various somatic elements such as glandular, neuronal, and cartilaginous tissue. However, typical extraembryonic elements, such as choriocarcinoma and yolk sac elements, were not usually observed. Production of hCG and/or AFP was identified in the tumors by both radioimmunoassay and the immunoperoxidase reaction. These results indicate that some human EC cell lines retain the capacity for differentiation into somatic and extraembryonic elements and that hCG and/or AFP appears to be produced by transitional types of cells that still retain the morphology of EC cells.