184 G. W. BARENDSEN 



concerning the effects of ionizing radiations on this system were re- 

 ported by Puck (1959). Using 150 kV X-rays he showed the capacity 

 for clone formation of HeLa cells to be very sensitive to this type 

 of ionizing radiation, with an LD37 of the order of 100 roentgens. 

 Hood et al. (1959) obtained a similar survival curve with 25 kV X- 

 radiation. 



Elkind and Sutton (1959) studied the survival of cells derived from 

 ovarian tissue and lung tissue of a Chinese hamster, after fractionated 

 doses of 55 kV X-radiation. They found a definite repair of accumulated 

 damage by surviving cells before their first post-irradiation division. 



In the experiments to be described in this paper the effect of ionizing 

 radiations of different linear energy transfer (LET) was studied on the 

 capacity for clone formation by kidney cells of human origin. The 

 survival ciu've was found to be exponential in the case of a-radiation 

 whereas for radiations of low LET a more complicated curve was 

 obtained. 



Experiments with fractionated doses showed that partial repair 

 takes place after X- and ^-ii-radiation but no recovery could be de- 

 tected after a -irradiation. Furthermore the effect was studied of 

 various doses of a-radiation followed by X-irradiation and conversely. 

 Finally it was shown that the effect of oxygen on the radiosensitivity 

 of these cells is much smaller for a-radiation than for X-radiation. 



MATERIALS AND METHODS 



In all experiments, kidney cells of human origin (van der Veen et al., 

 1958) were used, subcultured many times in glass bottles. The culture 

 medium consisted of Hank's solution with 0-5 per cent lactalbumin 

 hydrolysate and 5 jjer cent calf serum to which 100 lU penicillin and 

 0-1 mg streptomycin per ml were added. The incubator, at 37°C, was 

 continuously flushed with air, saturated with water vapour and con- 

 taining 3 per cent CO2 to maintain the pH of the culture medium at 7-4. 



The cells used in irradiation exjieriments were obtained from four 

 days old flask cultures in the proliferation ])hase. They were detached 

 from the glass and dispersed by gentle trypsinization (Puck et al., 1956). 

 The cell susj^ension obtained was counted in a haemocytometer. 

 Microscopic inspection showed that not more than a few per cent of the 

 cells were present in groups of two or three cells. 



The cells were plated on culture dishes, "conditioned" by l-5x 10^ 

 "feeder cells" in 3 ml medium which were made incapal)le of multipli- 

 cation by a dose of 4,000 rad of X-radiation (Puck et al, 1956). After 



