Finger nails are composed largely of alpha-keratin, a protein in which three right-handed alpha-helical peptide chains are twisted into a left-handed coil strengthened by disulphide cross links. We have found that the act of cutting nails generates trapped radicals, giving intense electron spin resonance (ESR) signals characteristic of sulphur centred radicals. The nature of the primary radical and the possible modes of generation are discussed. These radical reactions should be considered when using human nail parings to estimate accidental exposure to ionizing radiation.