Embryonal carcinoma cells (EC cells) can form a wide variety of differentiated cell types and thus resemble the pluripotential stem cells of the normal embryo. Certain EC cell derivatives acquire the biochemical and morphological features of primitive endoderm and have been called 'END' or endodermlike cells. Although these have also been called 'giant' because of their large size, their nuclear DNA contents are not known. Since cell size often corresponds to DNA content and primitive endoderm becomes polyploid during the course of normal development, EC-derived endoderm has been studied cytophotometrically. Thus, EC- and embryo-derived endoderm were found to be similar in that both of these tissues undergo polyploidization. Moreover, the polyploid cells of either EC or embryonic origin do not appear to be terminal cell types, since they can occasionally enter renewed cell division in spite of their large size.