NEW EVIDENCE ON THE MECHANISM OF RADIATION LEUKAEMOGENESIS 49 



If these results are substantiated, one would have to conclude that many 

 different tissues of the body are capable of yielding the transmissible factor in 

 response to radiation treatment. 



The next experiment was the crucial one — a repetition of the original in 

 vivo system, but using cell-free extracts instead of tissue mince. Only two 

 tissues — brain and blood plasma — were chosen, and the preparation of cell-, 

 free material consisted of four consecutive centrifugations at 7,000 x g. The 

 tissues were derived from animals that had been irradiated with 500 r 24 

 hours previously; the cell-free extracts were injected intraperitoneally into 

 normal recipients which, a week later, were submitted to ten weekly intra- 

 peritoneal injections of 20 mg of urethane. Two control groups received the 

 extracts without subsequent injections of urethane, and another control 

 group received urethane alone. 



This experiment is even less advanced than the previous one described 

 above, and only a few leukaemias have so far appeared. The results seem 

 encouraging, as will be noted from Table VI, but the final answer must 

 naturally await the completion of the experiment. 



Table VI. Incidence of leukaemia in C57BL/6 mice injected iviili cell-free 



extracts of tissues from irradiated mice, and recipients injected tvith urethane 



{Preliminary data: experiment in progress only 6 months) 



Irradiated donors received a single total-body exposure of 500 r. 



Cell-free extracts prepared by 4 centrifugations at 7,000 X ^: 1 ml c.f.e. of 



brain and 0-6 ml c.f.e. of plasma, resp., injected i.p, 

 Urethane treatment: ten weekly i.p. injections of 0-2 ml of 10% solution, 



totalling 200 mg per animal. 

 Interval between c.f,e, injection and 1st urethane injection: 1 week. 



It might nevertheless be worth considering at this stage, how far one may 

 venture to speculate, in the light of the results so far available, about the 

 relative merits of the five possible explanations, mentioned earlier, concerning 

 the presence of a virus in irradiated mice of a low-spontaneous-leukaemia 

 strain. 



The possibility that the promoting factor (e,g, urethane) operates on the 

 secondary target-organ (?the thymus), or indirectly on it via the bone- 

 marrow, rather than by some direct influence on the transmissible factor 



