It is now well established that when organic substances are irradiated in the solid state the observable result may be influenced by the presence of other compounds, demonstrating that a transfer of radiation energy occurs between substances in the solid state (1-8). Thus, by addition of small molecular substances, the degradation of polymethylmethacrylate can be reduced, and the inactivation of various enzymes may be either reduced or increased in the dry state. The nature of these interactions between irradiated substances in the solid state is as yet poorly understood. In a previous study of a molecular mixture of amino acids the intermolecular transfer of radiation energy was directly observed by electron spin resonance (ESR)3 spectroscopy (5). In the mixture of glutamic acid, glycine, and cysteine a slow transfer of radiation-induced unpaired spins to the cysteine sulfur was found. In the preceding paper (9), in which irradiated proteins were studied, evidence was obtained that the transfer of unpaired spins to cysteine sulfur atoms and to glycine residues occurs by mechanisms involving in part intermolecular processes. The purpose of the present paper has been to study by ESR methods the intermolecular transfer of radiation energy in mixtures of proteins with small molecular sulfur compounds which are protective in vivo. Only few ESR studies have so far been reported on mixtures of proteins with small molecular substances. Gordy and Miyagawa (4) found that when the maize protein zein was irradiated in the presence of small amounts of thiols the qualitative ESR spectra were different from that of either substance irradiated alone. It