Male, Kunming mice were irradiated by different doses of ,60,Co γ −rays (0—3Gy) in either acute or chronic radiation, and the hair, teeth and femora samples were studied with ESR spectrometer. While no characteristic ESR signals were found with the hair and teeth samples, ESR spectra of the femora were characterized by enhanced signal intensities with increasing dose of both acute and chronic exposures to the radiation. Storage condition of the irradiated femora samples was studied. No significant changes in the ESR signal intensities were observed with the samples kept dry at room temperatures for up to 453 days, whereas the intensity decreased significantly (,p,<,0.001) from the samples kept in formalin for just 7 days.