The effect of pellets of prosthesis materials on free radicals were examined using electron spin resonance spectrometry. Pellets used were titanium, alumina, ziruconia, an alloy of cobalt with chromium and polyethlene. These pellets were implanted in the abdominal skin and thigh muscle tissue of rats. Animals were sacrificed by decapitation at 1, 3, 20 hours and 2 and 4 weeks after implantation and subcutaneous and muscle tissues were dissected. Hydroxyl radicals and carbon centerd radicals were analyzed as spin adducts of spin trap, 5, 5-dimethyl-1-pyrroline-N-oxide. Hydroxyl and carbon centered radicals in the skin and muscle tissues were increased 1-3 hours after the implantation of these materials and these changes were almost recovered to the normal level 20 hours after the implantation except for alumina and ziruconia in the muscle. However, hydroxyl radicals and/or carbon centered radicals were increased in the skin 4 weeks after the implantation of zirconia, titanium, an alloy of cobalt with chromium and polyetylene. These results suggest that the subacute stage after the implantation of particles of prostheseis materials into the skin and muscle tissues induces the generation of free radicals and than these free radicals may produce tissue damage.