Abstract
RF-DNA of X174 is inactivated by X-rays and heat with a killing efficiency of about 0.25, suggesting that the genome is made up of critical and non-critical targets. It appears that one lesion alone in the critical target kills the viral activity, while surviving RF bearing several lesions only in the noncritlcal target can still restitute a normal replica of the viral genome. Restitution mechanisms are discussed.