PROTECTION AGAINST RUNTING 313 



had to be used in high doses in order to have a lethal action on 

 A newborns. 



The main interest of these results on immunotoxicity obtained 

 with minute amomits of specific antisera is to provide a solid 

 basis for the interpretation of the protective action of the same 

 sera (utilized at lower doses) in terms of immunological facilitation 

 — or specific enhancement. 



The last part of this work is concerned with results obtained in 

 the study of specific acquired tolerance of skin grafts. 



III. Increase in tolerance 



After having been followed for runting, all the survivors of the 

 two series of strain-A animals treated at birth with homologous 

 (CBA) cells (plus either normal or facilitating serum) were 

 grafted with corresponding homologous (CBA) skin and the 

 evolution of the grafts was observed. Following the criteria 

 described under "Methods", the grafted animals were separated 

 in 5 groups according to the evolution of the grafts. The groups 

 were dcfmed as follows : 



Group 0: animals which died between day o and day i6 after 

 grafting (lO animals) : these animals were discarded as of no use in 

 evaluating tolerance or rejection (no graft was rejected before 

 death in this group). 



Group I : animals which died between day 30 and day 100 (no 

 animal died between day 16 and day 30). All of them (10 animals) 

 had perfectly healthy grafts when they died : they were classified 

 as "undetermined tolerance", which means that it was impossible 

 to know whether they would have been cases of partial or per- 

 manent tolerance, had they survived. 



Group 2: animals which rejected their grafts before day 16 

 were classified as cases of "normal rejection". 



Group 3 : animals which rejected their grafts between day 30 

 (no animal rejected its graft between day 16 and day 30) and day 

 100 were classified as cases of "partial tolerance". 



