Quantitative observations on recovery from whole body irradiation in mice. II. Recovery during and after daily irradiation.

The recovery rate after repeated whole body irradiation has been examined: (1) by analysis of the total doses required to kill when irradiation is given daily at different levels of daily dose; (2) by determining the additional single dose required to kill at the end of a limited period of daily irradiation; (3) by determining the change in (2) during a radiation-free period after the end of a limited period of daily irradiation. The conclusions sometimes depend on the methods of analysis of the data but in general continued irradiation was found to damage progressively the ability to recover. Recovery rate and resistance to the killing effects of large single doses may vary independently. It is concluded that experiments using as their end point the direct lethal action of radiation are of little value in assessing long-term hazards. Animals which have received enough radiation to ensure death from late effects may show little impairment of resistance to single large doses.