Embryonic stem cells have the ability to remain undifferentiated and proliferate in vitro while maintaining the potential to differentiate into derivatives of all three embryonic germ layers. The aim of the present study was to establish mouse ES lines from blastocyst stage embryos obtained after CD1/EGFP mice superovulation. We isolated, cultured and determined the characteristics of mouse embryonic stem cells in early passages, which were first described by Evans M and Kaufman M. Therefore, we evaluated the morphological criteria for the approval of ES cells in early expansion stage. Two cell lines were isolated (CDE1 and CDE2) and analyzed. They showed similar characteristics to those reported earlier for blastocyst-derived ES cell lines.