Influence of Chemicals on Radiosensitivity 133 



of streptomycin are given and possibly some nutritional 

 support is supplied for the irradiated animals (Burnett and 

 Doherty, 1955). 



A few generalizations in regard to the mechanism of pro- 

 tection by these different treatments are in order. Chemical 

 protection is apparently quite general, affecting many 

 different functions of the organism. This is obvious from the 

 ability of the chemicals to protect against graying and some- 

 what against cataract formation, to maintain body weight, 

 and to reduce the number of glandular disturbances. Post- 

 treatment with spleen or bone marrow, apparently, will 

 stimulate the function of the haematopoietic systems. Many 

 interesting findings could be discussed here in regard to the 

 protection by posttreatment, especially the immunological 

 aspects. These may come to light in the discussion. It 

 should be pointed out that most of these studies emphasize 

 only the immediate effects. Very little is known in mammals 

 about the effect of radiation protection on the long-delayed 

 effects; i.e., late-appearing malignancies and mutational 

 changes. Actually, these latter ones may prove to be the most 

 important ones. In bacteria, the situation is somewhat 

 clearer since genetic effects in micro-organisms may be readily 

 observed. This brings up one of the most important questions 

 in modern radiobiology ; namely, "Is the genetic damage 

 entirely dependent on the amount of ionizing radiation to 

 which the cell is exposed, or can the effect be modified by 

 chemical protection or by treatment after the exposure has 

 ended? " Bacterial cells are very well suited for checking this 

 since the chemical used will penetrate to most parts of the 

 cell and, after treatment, will help a large number of cells 

 to survive. In a number of strains of Esch. coli it has been ob- 

 served that, for certain mutations (nutritional reversions), the 

 mutation rate increases inversely to the survival ratio and is 

 not necessarily proportional to the amount of radiation to 

 which the cells are exposed. This means that the dose- 

 reduction factor for mutations approaches the DRF for 

 survival. In other words, it is possible in certain strains of 



