This study was designed to investigate the increase in the number of circulating CD34+ cells after acute myocardial infarction (MI) and the differentiation of these cells to cardiomyocytes after transplantation into infarcted myocardium. The study involved five donor groups: MI (n=27), sham (n=26), granulocyte-colony stimulating factor (GCSF) (n=26), MI+GCSF (n=25), and control (n=25). Acute MI was induced by ligating the left anterior descending coronary arteries (LAD) of donor rats, and LAD of recipient rats were ligated on the same day. Seven days after ligation, CD34+ cells in donor rats were counted and then were directly injected into the infarcted myocardium of recipient rats. Eight weeks after the transplantation, significant differences (p<0.001) were observed in the CD34+cell counts among the 5 donor groups with the greatest increase in the MI+GCSF donor group. In rats receiving CD34+ cells, the size of the scar area smaller (p<0.001) and the thickness of the scar was greater (p=0.001) than in CD34- and saline-transplanted rats. The transplanted CD34+ cells differentiated into cardiomyocytes in the scar. This study suggests that CD34+ cells may be a potential source of stem cells and that they may be useful in strategies aimed at the regeneration of infarcted myocardium.