The results reported in this article show that although erythropoiesis which occurs in organotypic cultures of the established PCC 3/A/1 teratocarcinoma cell line is restricted to the primitive erythrocytic series, stem cells able to differentiate along the definitive erythrocytic lineage are present in the cultures. These precursors can express their potentialities when culture fragments are grafted into the chick embryo. A possible explanation could be found in the lack of liver tissue differentiation in in vitro cultures of this teratocarcinoma line. In contrast, one can see in the grafted teratocarcinoma, a tissue showing the structure of liver parenchyme and associated with numerous anucleated erythrocytes.