The effects of γ radiation on glycylglycine, triglycine, and polyglycine have been studied using ESR spectroscopy. The radiation exposures have been performed under vacuum at room temperature. All those irradiated substances show a doublet absorption signal often ascribed to a radical formed on the α carbon of the peptide chain. The magnetic susceptibility has been found to depend on temperature according to the Curie-Weiss law. The radio-sensitivity parameterGr, the hyperfine splitting constantAhand the spectroscopic factorgnave been determinated as well as which is the temperature corresponding to the Curie temperature of the irradiated substance. The results onCr, andAhfor all peptides studied here agree with the assumption of radiation induced free radicals located at the α carbon of the peptide chain, leading to conclude that the degree of delocalization of the unpaired electron increases for increasing number of glycine units in the peptide. Finally, from negative values obtained, a conclusion has been reached on the antiferromagnetic nature of the interaction between unpaired spins.