The secondary processes occurring in proteins X-irradiated in the dry state were followed by electron spin resonance (ESR) spectroscopy. The formation of sulfur radicals was given particular attention. Crystalline proteins were irradiated in vacuum at 77[degree]K. Uncharacteristic resonance patterns were observed, reflecting the existence of several kinds of ESR center, with no sign of sulfur resonance. On heat-treatment the characteristic glycylglycine pattern and/or sulfur pattern found after irradiation of proteins at room temperature appeared. When the samples were heat-treated, the total number of spins first decreased and subsequently increased significantly. The formation of sulfur radicals, which appears at slightly below room temperature, coincides with the increase in the total number of radicals,, Quantitative studies on a number of proteins indicated that the extent of sulfur resonance formed at room temperature depends on the ratio cysteine-cystine/glycine. With modified proteins (thiolated gelatin) more sulfur resonance was formed when the sulfur was present as sulfhydryl groups than when in the disulfide form. The radical yield for proteins was of the same order of magnitude as that for a number of amino acids. Heat-denaturation of proteins in solution did not significantly alter the yields. The radical yields at 295[degree]K were consistently higher than when the substances were irradiated at 77[degree]K and subsequently annealed to 295[degree]K. It is suggested that free radicals induced in proteins are in part formed by homolytic rupture of chemical bonds and that unpaired spins are transferred to specific structures partly by inter-molecular processes.