IMMUNOGENETICS OF TUMOURS IN CHIMERAS 167 



metastasis only in an isologous system, i.e. when the tumour, the 

 inoculated haemopoietic cells, and the irradiated host, were of the 

 same isoantigenic constitution (Table II, groups i and 2). No 

 metastasis appeared when sarcoma Cio was grafted in homologous 

 chimeric hosts, i.e. in irradiated C57BL mice protected by C57BL 

 cells (Table II, groups 3 and 4). It should be noted that in homo- 

 logous hosts produced by the injection of foetal C57BL cells into 

 C57BL mice, the primary subcutaneous transplant of Cio grew 

 progressively (Tables I and II), yet no metastasis appeared. On the 

 other hand, sarcoma MCiM showed a regular progression of 

 metastasis in homologous chimeric hosts produced by foetal 

 C57BL cells inoculated into C57BL mice (Table II, group 7). 

 Homografts of sarcoma SBLi, produced in C57BL animals by 

 3 : 4-benzpyrene, showed progressive growth of the original 

 transplant in the two groups of homologous chimeric hosts, i.e. 

 in irradiated C3H mice protected by C3H spleen cells, and in 

 irradiated C3H mice protected by foetal C3H cells (Table III). 

 Yet metastasis developed only in the homologous animals pro- 

 tected by foetal cells (Table III, group 4). A spontaneous mammary 

 adenocarcinoma, S2, which developed in a Swiss mouse in our 

 colony, did show the formation of metastasis in both types of 

 homologous chimeras: in irradiated C3H protected by C3H foe- 

 tal cells, and in irradiated C3 H protected by C3 H spleen cells (Table 

 III, groups 10 and 11). This tumour, unlike the previous ones, 

 gave successful homografts when transplanted subcutaneously 

 to normal, non-irradiated C3H hosts. It thus appears that in 

 addition to the effects of total body irradiation, which promoted 

 the formation of metastasis in isologous systems, certain immuno- 

 logical conditions, manifested in homologous systems, are among 

 the determining factors in a host-tumour relationship which 

 control the development of metastasis. Lymph node metastasis 

 seems to be more susceptible to the immune reactions in incom- 

 patible chimeras than the primary transplant. Thus, a Cio 

 homograft in foetal cell chimeras grew progressively, yet no 



