48 I. BERENBLUM AND N. TRAININ 



within 24 hours of radiation. This raised the question of whether the factor 

 might actually be a virus or precursor- virus type of entity. 



The fact that all the tissues tested appeared to be equally effective, could 

 mean one of two things: (i) that the "target-organ" for the liberation of the 

 transmissible factor is widespread in the body, or (ii) that the site of formation 

 may be restricted to one organ, but that the liberated agent is rapidly 

 dispersed throughout the body. 



Table V. Incidence of leukaemia in C57BL/6 mice injected with tissues irra- 

 diated in vitro and subsequently injected ivith urethane {Preliminary data: 



experiment in progress only 8 moyiths) 



Donor tissues irradiated in vitro with 1,000 r. Urethane to recipients: ten weekly i.p. injections 

 of 0-2 ml of 10% solution, totalling 200 mg per animal. 



To find out which of these two explanations is the correct one, a further 

 experiment was undertaken, essentially the same as the previous one, except 

 that the various tissues, taken from normal animals, were irradiated in vitro. 

 Other modifications in technique included the following: {a) dose of in vitro 

 radiation: 1,000 r; (6) tissues used: brain, liver, kidney, spleen, thymus, 

 lymph nodes, bone-marrow and lung; and (c) urethane treatment to the 

 recipients: 10 weekly doses, totalling 200 mg per animal. 



The experiment has only been in progress about 8 months, and the figures, 

 to date, provide no more than an indication of the trend of the experiment. It 

 will be noted, however, that all the leukaemias so far recorded (see Table V) 

 belong to the series in which injections of irradiated tissues were followed by 

 urethane treatment. In our urethane controls, i.e. without prior tissue 

 injections, the leukaemia incidence for the equivalent, short, time of action 

 was 0-5%. 



