The changes in mechanical properties of poly(methyl methacrylate) (PMMA) and glass fibre reinforced plastic (GFRP) induced by high energy (30, 45 MeV) protons were studied and compared with those induced by Co-60 gamma rays. The flexural strength at break of PMMA and GFRP showed the same degradation behaviour as a function of dose, indicating no difference between proton and Co-60 gamma ray irradiation.