This study describes a method to generate pure cultures of cardiomyocytes from differentiating murine embryonic stem (ES) cells. A fusion gene combining the α-cardiac myosin heavy chain promoter and aminoglycoside phosphotransferase cDNA was stably transfected into ES cells. After in vitro differentiation and G418 selection, the resulting cardiomyocytes were highly differentiated, with over 99% purity. These cells were tested for their ability to form stable intracardiac grafts in dystrophic mice. The engrafted cells were monitored using immunohistology and PCR, showing presence for up to 7 weeks. The study indicates that genetic manipulation can produce essentially pure cardiomyocyte cultures from ES cells, which are suitable for intracardiac grafting. This approach is applicable to all ES-derived cell lineages. The selected cardiomyocytes exhibited spontaneous and rhythmic contractile activity, and formed stable grafts in the hearts of dystrophic mice. The results suggest that ES-derived cardiomyocytes can be used as a viable alternative to fetal donor cells for cardiac engraftment. The study also highlights the potential of ES-derived cells for cardiac repair and the importance of developing reliable methods for generating pure cardiomyocyte cultures. The findings have implications for future therapeutic strategies in cardiac disease.This study describes a method to generate pure cultures of cardiomyocytes from differentiating murine embryonic stem (ES) cells. A fusion gene combining the α-cardiac myosin heavy chain promoter and aminoglycoside phosphotransferase cDNA was stably transfected into ES cells. After in vitro differentiation and G418 selection, the resulting cardiomyocytes were highly differentiated, with over 99% purity. These cells were tested for their ability to form stable intracardiac grafts in dystrophic mice. The engrafted cells were monitored using immunohistology and PCR, showing presence for up to 7 weeks. The study indicates that genetic manipulation can produce essentially pure cardiomyocyte cultures from ES cells, which are suitable for intracardiac grafting. This approach is applicable to all ES-derived cell lineages. The selected cardiomyocytes exhibited spontaneous and rhythmic contractile activity, and formed stable grafts in the hearts of dystrophic mice. The results suggest that ES-derived cardiomyocytes can be used as a viable alternative to fetal donor cells for cardiac engraftment. The study also highlights the potential of ES-derived cells for cardiac repair and the importance of developing reliable methods for generating pure cardiomyocyte cultures. The findings have implications for future therapeutic strategies in cardiac disease.