Effects of ionizing radiation on 18 polyamino acids were examined with the electron spin resonance (ESR) method. In about half of these it was possible to interpret the ESR observed at room temperature as arising from free radicals formed by loss of the H from the [long dash] CHR[long dash]group of the polypeptide chain. In the others, the room-temperature ESR signals seem to arise from a damaged side chain or R group. The ESR obtained at 77[degree]K were generally different from those obtained at room-temperature and may arise often from charged or ionized molecules. Oxygen altered or quenched the ESR of all the polyamino acids studied, some more rapidly than others. The alteration of the signal is thought to be caused by a conversion of the radical X to peroxide radicals X[long dash]O[long dash]O. The later decay of the signal probably resulted from the release of H atoms, which escape through the lattice, and the consequent conversion of the polymeric free radicals to stable molecules.