Recent studies reported the use of bone marrow-derived cells as a renewable and readily accessible source for generating cardiomyocytes. We isolated the highly enriched, the so-called ���side population(SP) cells��� from bone marrow by Hoechst 33342 dye efflux to identify a source of stem cells capable of cardiac regeneration. We cocultued SP cells with neonatal heart cells whether they have a potency to differentiate into cardiomyocytes in vitro. Cocultured Dil-labeled SP cells with neonatal cardiomyocytes expressed sarcomeric-a-actinin not in cultured only SP cells after 5-days. We transplanted 5000 Dil-labeled SP cells isolated bone marrow of mice (4 to 5 weeks of age) into border zone of infarcted myocardium after ligation of LAD coronary artery. Implanted SP cells formed DiI-positive engrafts in infarcted myocardium and stained with Sarcomeric a-actinin and myosin heavy chain two weeks after cell transplantation. SP cells are highly enriched cells to differentiate into cardiomyocytes both in vitro and in vivo, which suggests that SP cells derived from bone marrow are one of important cell sources to regenerate the damaged cardiomyocytes.