On annealing at temperatures near 100C, carbon-centered radicals migrate to sulfur-centered radicals in X-irradiated crystals of L-a-amino-n-butyric acid hydrochloride, CH3CH2CH(NH3C1)COOH, containing L-cysteine hydrochloride, SHCH2CH(NH3C1)COOH. Samples containing 0, 0.5, 1.0, and 1.5% L-cysteine hydrochloride were studied. When no cysteine is present, the carbon-centered radical formed by X irradiation, CH3CH2CHOOH, decays according to a second-order diffusion-controlled rate equation. In samples containing cysteine, the same carbon-centered radicals are formed, but on annealing, they migrate to cysteine, where a perthiyl radical, RSS*, is formed. The transfer of carbon-centered radicals to perthiyl radicals follows a pseudo first-order rate equation with an activation energy of 1.15 eV. A decrease in the initial concentration of the carbon-centered radicals or an increase in the initial concentration of cysteine results in an increase in the transfer efficiency. The rate of growth of the perthiyl radical depends on both the initial concentration of cysteine and the initial concentration of carbon-centered radicals. The pseudo first-order rate constant increases when either the initial carboncentered radical concentration increases or the initial cysteine concentration increases. The mechanism by which radicals move from one lattice site to another in the crystalline material is most likely hydrogen abstraction from a neighboring molecule.