We have studied molecular interactions in irradiated solids by examining the electron paramagnetic spectra of various molecular mixtures and by determining the effect of glutathione on the X-ray inactivation of catalase. The spectroscopic data show that direct electron transfer can occur between molecules in irradiated solids. Such electron transfer may be the mechanism in the interaction of polymethylmethacrylate with thiourea and other compounds (1), in the interaction of glutathione and ribonucleic acid (2), and in the energy transfer between enzyme and substrate (3, 4). Whatever the mechanism, these interactions between molecules in the solid state are of particular interest in biology, since so much of the cellular apparatus is contained in solid particles.