172 MICHAEL FELDMAN AND DAVID YAFFE 



The first factor might exert its effect through an antigen- 

 antibody interaction, which could result in an immunoselection 

 of compatible cells. The second points to some interactions 

 between cells of different antigenic constitution. Since radiation 

 chimeras of various genetic combinations can be constructed, this 

 system can be used to differentiate between the relative importance 

 of these two assumed factors, i.e. between the function of the low 

 immune pressure on the tumour cell population, and the function 

 of the antigenic "foreignness" of the host. 



A series of experiments was carried out in which SBLi was 

 grown for 27 days in the following chimeric animals, and then 

 tested for acquired homotransplantability in C3H animals: 



(i) Irradiated C3H treated with foetal C3H cells. 



(2) Irradiated C3H treated with foetal C57BL cells. 



(3) Irradiated Fi(C3H x C57BL) treated with foetal C3H cells. 



(4) Irradiated Fi(C3H x C57BL) treated with foetal C57BL 



cells. 



The criteria for the evaluation of the intensity of the change in 

 each of the experimental groups, were (a) the number of lethal 

 takes of the SBLi in the secondary non-irradiated C3H hosts, (b) 

 the growth period of SBLi in the secondary C3H prior to re- 

 gression (in those cases which did not give lethal takes), and (c) the 

 maximal size attained by the tumour homograft in C3H animals. 



The results (Feldman and Yaffe, 1959) showed that the most in- 

 tense change takes place in C3H-^C3H chimeras. The C57BL-> 

 C3H were also found to be good inducing hosts, although to a 

 somewhat lesser degree than the C3H->C3H animals. Tumours 

 growing in the C3H->Fi(C3H x C57BL) chimeras showed a 

 prolonged survival in the secondary normal C3H, but without 

 lethal takes. A definite change towards homotransplantability 

 was observed, but homotransplantable sublines were not estab- 

 lished. The C57BL-^Fi(C3H x C57BL) were the weakest 

 ** inducing" hosts. 



