NEW EVIDENCE ON THE MECHANISM OP KADIATION LEUKAEMOGENESIS 



47 



It will be noted that the untreated controls remained free from leukaemia, 

 while those treated with urethane alone, or with non-irradiated tissues plus 

 urethane, developed leukaemia in about 5% of animals. The mice receiving 

 irradiated tissues plus urethane, on the other hand, yielded about 20% 

 leulvaemia, the type of tissue used being apparently not a decisive factor, nor 

 the interval between the radiation of the donor mice and the time of transfer 

 of tissues to the recipients. In the groups receiving tissues from irradiated 

 mice ivithout subsequent urethane treatment, the incidence of leukaemia was 

 very low indeed. 



Table IV. Incidence of leukaemia in C57BL/6 fnice injected ivith tissues from 

 irradiated and non-irradiated donors, and subsequently with urethane (jilus 



controls) 



Total 51/262(19%) 3/334(1%) 46/269(17%) 8/348(2%) 16/305(5%) 



Controls: Urethane alone (without previous tissue injections) 5/110 (5%) 

 Untreated controls 1/141(0-7%) 



The results suggest that total-body irradiation, in mice of a low- leukaemia 

 strain, causes the rapid appearance of a "transmissible" factor which, 

 though itself not capable of transmitting the disease to other mice, does 

 render such recipients subject to leukaemia formation by urethane treatment 

 over and above that which can be accounted for by the action of urethane alone. 



The possibility of the "transmissible" factor being a simple chemical 

 substance liberated by the radiation, can be ruled out, since the agent 

 appears to be present in as effective a concentration after 30 days as after 1 

 day following the radiation. The possibility that the "transmissible" factor 

 is a dormant tumour cell, is also rendered unlikely by the fact that it is 

 distributed throughout all the tissues of the body, which have been tested, 



