Human peripheral blood lymphocytes were exposed to 300 rad gamma-rays, in the presence or absence of radical scavengers, and the change in the frequency of chromosome aberrations was analysed with attention directed to the protection by scavengers against the formation of primary damage leading to chromosome aberrations. The results showed that the damage involved in the formation of exchange-type aberrations was efficiently protected by scavengers, and about 60 per cent of them resulted from indirect action that could be abolished by alcohols. The SH-compounds afforded additional protection. The comparison of protective ability with reaction rates demonstrated that the indirect effect was due to the reaction of OH radicals possibly to DNA as a target molecule, and the involvement of H and eaq-was unlikely. In contrast to the exchange-type aberrations, terminal deletions were not significantly protected, suggesting that the damage leading to the terminal deletion differed in its nature from the leading to the exchange-type aberration.