The transplantable tumour line derived from a spontaneous ovarian murine teratocarcinoma (Fekete & Ferigno, 1952) was cloned and characterized using light and electron microscopic and immunohistochemical techniques. Grown in ascites, the tumour consisted predominantly of stem cells and a small number of differentiated derivatives. The stem cells expressed surface reactivity with antibody to SSEA-3 and Forssman antigen, alkaline phosphatase, focal cytoplasmic reactivity with antibody to SSEA-1, and varying amounts of cytoplasmic glycogen and 3 beta-hydroxysteroid dehydrogenase. Their cytoskeleton reacted with antibodies to keratin and vimentin. The differentiated derivatives formed approximately 5-15% of the total cell population in ascites and appeared either as giant cells or were characterized by their reactivity with antibodies to H-2 or alpha-foetoprotein or intracellular and pericellular laminin or high levels of 3 beta-hydroxysteroid dehydrogenase activity. Solid tumours produced from subcutaneously injected cells had a variegated appearance suggesting, that like the limited differentiation in the ascites, the stem cells can give rise to trophoblastic, as well as parietal and visceral yolk sac elements. On the basis of the presented data the tumour stem cells were considered as representing malignant equivalents of the common precursor of trophoblastic, visceral and parietal yolk sac cells most likely corresponding to trophectoderm. Accordingly, the tumour was designated as trophectodermal carcinoma.