190 



G. W. BARENDSEN 



significant differences in effect were observed if the order in which a- 

 and X-irradiations were given was reversed. It will be clear from curves 

 1 and 2 that if a certain dose of a-radiation is given first, the curve 



1003 



I 23456789 10 

 Dose (rad x 100) *- 



Fis 3 —Effects of combined a- and 200 kV X-radiation on the capacity for clone for- 

 ^' ' niation. 



fl ^nd b Effects of OL- and 200 kV X-radiation respectively. 



CiSve 1, Effects of 50 rad a-radiation + 0; 100; 150; 200; 300 and 500 rad of 200 kV 



X-radiation. 

 Curve 2. Effects of 100 rad a-radiation + : 100; 150; 200: 300 and 500 rad of 



200 kV X-radiation. 

 Curve 3 Effects of 300 rad 200 kV X-radiation + (); 50 and 100 lad a-radiation. 

 Curve 4'. Effects of 500 rad 200 kV X-radiation + ; 50 and 100 rad a-radiation. 



obtained from X-irradiation of the surviving cells has the same shape 

 as if no preceding a-irradiation had occurred, i.e. X-radiation acts on 

 cells surviving a-irradiation as if they were not irradiated at all. This 

 contrasts with the effect of two doses of X-radiation where the damage, 

 at least in part, is cumulative. The dotted lines 3 and 4 parallel to the 

 survival curve a obtained with a-radiation indicate that, within limits 

 of error cells surviving after X-irradiation have not become more 

 sensitive to a-radiation i.e. in this respect there is no cumulative effect. 

 This supports the conclusions given in the first section. 



Effects of different oxygen concentrations on radiosensitivity 

 A reduction in the sensitivity to ionizing radiation resulting from 

 anoxia has been reported for many systems (Bora, 1958; Gray et al, 

 1958- Neary et al, 1959). In order to measure the effect of different 

 oxygen concentrations on the capacity for clone formation, the medium 

 was removed from the culture dishes after the cells had attached to the 



