The objective of this study was to screen mouse bone marrow mesenchymal stromal cells (BMSCs) according to expression of cardiac stem cell (CSC) surface antigens and to assess the effects of resulting BMSC-like subsets on cardiac function after injection in a mouse myocardial infarct model. BMSCs were sorted by magnetic beads according to the expression of differentiation antigens on the surface of mouse CSCs, and four subsets were identified on the basis of CD45 and CD31 expression: stem cell antigen-1+ (Sca-1+)/CD45-/CD31-, Sca-1+/CD45-/CD31+, Sca-1+/CD45+/CD31-, and Sca-1+/CD45+/CD31+. When co-cultured with myocardial stem cells and 5-aza-2'-deoxycytidine for 14 days, each subset showed expression of cardiac markers α-actin, connexin 43, desmin, and cardiac troponin I; however, expression was greatest in Sca-1+/CD45+/CD31+ cells. To assess the ability of these cells to improve cardiac function, each subset was injected separately into mice with myocardial infarct induced by ligation of the left anterior descending coronary artery, and in vivo cardiac dual inversion recovery (DIR) imaging and Doppler echocardiography were performed 48 h, 96 h, and 7 days after injection. Results indicated that Sca-1+/CD45+/CD31+ cells were superior in improving cardiac function compared with the other subsets and with unsorted BMSCs. These results suggest that mouse BMSC cells are polyclonal and that the BMSC-like Sca-1+/CD45+/CD31+ subset was effective in directing cardiac differentiation and improving cardiac function in mice with myocardial infarcts.