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Several cell lines (either of embryonal carcinoma or of differentiated cells derived from teratomas) have been established in vitro from transplantable testicular teratomas. Primitive cell lines, propagated in vitro as embryonal carcinoma have retained the ability of the original tumor to differentiate in vivo or in vitro into must embryonic cell types. Features of this model system for study of early embryogenesis are described. Emphasis is placed on the description of the cell surface antigens of several cell lines. Syngeneic antisera raised against two primitive lines (F9 and PCC4) and against a differentiated one (Endo) have allowed the detection of three groups of cell surface antigens, present on teratoma cells, tumor cells and embryonic cells. The F9 antigen appears to be specific to be specific of the very early steps of egg development (morula and blastocyst). After egg implantation, it keeps expressed on the cells of the male germ line. The PCC4 antigen has a similar cell type distribution but appears to be more specific of multipotential cells. The Endo antigen is essentially specific of endodermal derivatives. The F9 antigen is probably specified by the wild type allele (+ tl2) of the tl2 gene at the T-Locus of the mouse, a gene which plays some critical role in early development. The molecular structure of this antigen, as determined from immunoprecipitates is very similar to that of H-2 antigens. In addition, a cross-reacting material is found in Man, with a tissue distribution identical to that found in the mouse.