Background : There have been known that adult somatic tissues contain stem cells such as side population cells, that can be isolated with fluorescence-activated cell sorter. For example SP cells were previously isolated from bone marrow and skeletal muscle. Recently a few reports have suggested that adult murine hearts also contain potential cardiac stem cells. However, it remains to be clarified whether these cells have the characteristics of stem cells such as other SP cells such as bone marrow SP cells. Here, we characterize adult heart-derived SP cells(CM-SP), compare to bone marrow-derived SP cells(BM-SP) and investigate plasticity. Method and Results : Single-cell suspensions of murine hearts were stained with Hoechst 33342 dye and were characterized their hematopoietic cell surface markers by FACS analysis, real time RT-PCR, and colony forming assay. CM-SP existed at a frequency of 0.03 % to 0.1% in the cell suspension of the digested heart. The vast majority of CM-SP (n = 5) showed primitive cell phenotype such as Sca-1+, c-kit+, and CD45+ (91.9 짹 1.73 %, 9.6 짹 4.1 %, and 8.3 짹 0.6 %, respectively). According to real time RT-PCR, CM-SP highly expressed early endothelial progenitor cell marker such as Flk-1, Tie-2 than [Beta]-myosin heavy chain, desmin, suggesting the potency of angiogenesis. CM-SP also contained lower but significant levels of hematopoietic colonies similar to BM-SP (4.1 짹 1.2 and 112 짹 21.3 per 1 x 10⁴ nucleated cells, respectively).CM-SP cells were phenotypically distinct from bone marrow SP cells. DiI-labled CM-SP cells were intramuscularly implanted into the ischemic limbs model of murine. Laser Doppler imaging revealed markedly increased blood perfusion in CM-SP-implanted limbs compared with control. CM-SP cells differentiate into endothelila and skeletal muscle cells. These results provide an indication of cardiac SP plasticity. Discussion: Adult heart-derived SP cells expressed highly early angiogenic markers, which suggest angiogenesis rather than myogenesis may play an important role in the functional improvement after cell transplantation.