LEONARD J. COLE AND MARIE E. ELLIS 



against otherwise lethal whole-body X-irradiation (725r). The data 

 indicate, furthermore, that protective activity of such homogenates is associ- 

 ated with the nuclear fraction obtained by differential centrifugation ; the 

 mitochondria and supernatant subfractions exhibit no protective activity, 

 under the conditions of these experiments. 



♦ Marrow removed 2 days after 750 r. 



The data in Table I also show the high radiosensitivity of the recovery 

 factor in rat marrow. Thus, the ability of rat bone marrow homogenates 

 to afford protection against acute radiation death is annulled by in vitro 

 irradiation of such homogenates with 725 r. (250 kvp X-rays, approxi- 

 mately 25 r per min). Furthermore, marrow taken from X-irradiated rats 

 (725 r whole-body irradiation) 48 hours after exposure, no longer exhibits 

 any protective activity. It appears therefore that the radiation recovery- 

 factor present in rat bone marrow homogenates possesses at least some 

 properties similar to that of the protective principle in mouse spleen 

 homogenates. 



In the light of the considerable body of evidence in the literature indicating 

 that recovery from whole-body X-irradiation injury at lethal dose levels is 

 associated with haematopoietic regeneration, it is of interest to compare 

 bone marrow and spleen from various animal species and strains as sources 

 of the recovery factor, and as to haematopoietic function. It is known that 

 homologous bone marrow is a more effective source of recovery factor than 

 homologous spleen. This is true for the LAf^ mouse^* as well as for the 

 Sprague-Dawley rat^^. In some strains of mice, furthermore, the spleen 

 does not provide a source of the recovery factor, whereas the bone marrow 



145 L 



