Electron-spin resonance spectra of X-irradiated synthetic apatites showed that the signals due to O^- radical are strongly dependent upon the carbonate substitution in the apatite structure. The disappearance of hyperfine interaction with an adjacent hydroxyl proton when the carbonate incorporation increases indicated that, in human enamel, the major portion of carbonate probably occupies phosphate sites and contributes at least for ~20 per cent of hydroxyl deficiency.