Using EPR spectroscopy it was established that the determination of the concentration of paramagnetic centers in lyophilized tissues allows indirect evaluation of the quality of decellularization of intrathoracic organs (diaphragm, heart, and lungs), since the content of paramagnetic particles in them can serve as a criterion of cell viability and points to the necessity to repeat decellularization. Experiments in rats showed that the EPR spectra of the native thoracic organs contained paramagnetic centers with g-factor values ranging from 2.007 to 2.011 at a concentration of 10(-8) to 6.62 × 10(-7) mol/g of lyophilized tissue, whereas in all decellularized tissues of the same organs paramagnetic particles were not detected.