We report the labeling (internalization) of skeletal myoblasts with a novel class of oxygen-sensing paramagnetic spin probe for noninvasive tracking and in situ monitoring of oxygenation in stem cell therapy using electron paramagnetic resonance (EPR) spectroscopy. Skeletal myoblast (SM) cells were isolated from thigh muscle biopsies of mice and propagated in culture. Labeling of SM cells with the probe was achieved by co-incubating the cells with submicron-sized (270±120 nm) particulates of the probe in culture for 48 h. The labeling had no significant effect on the viability or proliferation of the cells. The SM cells labeled with the probe were transplanted in the infarcted region of mouse hearts. The engraftment of the transplanted cells in the infarct region was verified using MY-32 staining for skeletal myocytes. The in situ pO₂ in the heart were determined noninvasively and repeatedly for 4 weeks after transplantation. The results showed significant enhancement of myocardial oxygenation at the site of cell transplant compared to untreated control. In conclusion, labeling of SM cells with the oxygen-sensing spin probe offers a unique opportunity for the noninvasive monitoring of transplanted cells as well as in situ tissue pO₂ in infarcted mouse hearts.