gamma-ray irradiation of an N-acetyl-L-tyrosine single crystal produces three stable radicals (A-C) which have been identified and characterized in their hyperfine coupling (hfc) tensors by using conventional and high-frequency electron paramagnetic resonance (HF-EPR) and X-band electron nuclear double resonance (ENDOR). Radical A is a neutral phenoxyl radical of which the g-tensor principal value corresponding with a good approximation to the C-O(H) direction is g(x) = 2.0094 +/-0.0002. The comparison with the corresponding values obtained for the similar tyrosyl radicals playing a role in different biological systems supports the idea of using the g, value as a probe to find the presence of a hydrogen-bond interaction involving the phenoxyl oxygen. Radical B is a neutral cyclohexadienyl radical obtained by hydrogen atom addition at a position ortho to the phenolic group of the tyrosine moiety. Its hfc tensors have been fully characterized and found to be in agreement with present theories. Radical C is found to be produced by reduction of the carboxylic group of the parent tyrosine derivative.