46 I. BERENBLUM AND N. TRAININ 



strain (C3H) iii which the disease was induced by radiation; and (iii) the obser- 

 vation by Lieberman and Kaplan (1959) that such an agent can be detected 

 in irradiated C57BL mice shortly before the induced leukaemia becomes 

 clinically demonstrable. 



There are at least four possible ways of interpreting these results: (i) that 

 the virus is already present in a low-leuhaemia strain of mice, but that the 

 target-organ (? thymus) is in a non-responsive state, and that radiation causes 

 the target-organ to become responsive; (ii) that a leukaemia virus is present in 

 insufficient amounts, and that radiation causes it to reach an effective con- 

 centration, possibly by neutralizing the action of inhibitors; (iii) that mice of 

 a low-leukaemia strain carry a precursor- virus, and that radiation converts it 

 into an active virus; and (iv) that the radiation actually produces a complete 

 leukaemia virus de 7iovo. In the light of the two-stage mechanism, a further 

 possibility needs to be considered^ namely (v) that initiating action (e.g. by 

 low doses of radiation) causes the liberation of a precursor- virus de novo, and 

 that promotmg action (e.g. by urethane) causes it to be converted into an 

 active virus. 



In an attempt to analyse these possible interpretations, use was made of 

 the radiation-m-ethane two-stage system in a modified form, whereby the 

 action of radiation and that of urethane might be tested in separate animals. 

 Any transmissible factor — virus or precursor-virus, liberated or already 

 present in the irradiated animals — could thus be transferred to normal anunals, 

 which could then be submitted to urethane treatment. Several large-scale 

 experiments of this kmd, with variations in technique, are in progress, some 

 of which have already yielded definitive results. 



In the first experunents, several hundred adult C57BL/6 mice were given 

 a single total-body exposure of 400 r; 24 hours later, the animals were killed, 

 and 7 tissues (bone-marrow, Ipnph nodes, spleen, thymus, lung, brain and 

 skin) were excised and minced, and each injected into separate batches of 

 adult C57BL/6 mice (about 50 mice per batch). One week later, urethane 

 treatment was given to the recipients — a series of 15 weekly intraperitoneal 

 injections, totalling 300 mg per animal. Controls included: {a) similar minced 

 tissue transfers without subsequent urethane treatment to the recipients; 

 (6) transfer of minced tissues from non-irradiated animals, followed by 

 urethane treatment to the recipients; (c) urethane treatment without previous 

 tissue injection; and (d) untreated controls. 



A second experiment, identical with the first, except that the interval 

 between the irradiation and the excision of tissues for transfer was 30 days 

 instead of 24 hours, was set up at the same time. 



The results of these two experiments, almost complete, of which a pre- 

 liminary report was previously published (Berenblum and Trainin, 1961b), 

 are summarized in Table IV. 



