INDUCTION OF TOLERANCE BY PARABIOSIS 329 



induced in lethally X-irradiated adult animals treated with homo- 

 logous or heterologous bone marrow respectively. 



For the past six years we have been investigating the possibility 

 of inducing specific tolerance to skin homografts in immuno- 

 logically mature rats by the method of parabiosis. 



A short survey of results obtained in Y->W strain 

 combination 



The attempts to maintain the parabiotic union in the Y->W 

 strain combination to the 20th postoperative day or beyond 

 proved unsuccessful (Nakic et al, 1961). Pairs were lost either 

 through spontaneous separation of parabionts or because of the 

 death of one of the partners as a result of" parabiosis intoxication" 

 (see Finerty, 1952). Studies of the effect of the duration of the para- 

 biotic union on the incidence of tolerance revealed that parabiosis 

 of less than five days' duration was ineffective in producing toler- 

 ance of an appreciable degree. Best results were obtained with 

 parabiosis lasting six days; prolongation of the parabiotic state 

 to seven days or more did not bring about an improvement in the 

 results. A possible explanation of this has been discussed elsewhere 

 (Nakic et ah, 1961). 



Homografts exchanged between homologous rat parabionts 

 separated after five to six days survive longer than in control 

 animals (Nakic, Nakic and Silobrcic, i960). The same is true when 

 grafts from the third strain are transplanted into homologous 

 parabionts, showing that this depression of the immune reaction 

 might be non-specific. That the trauma involved in the para- 

 biotic procedures is an unlikely cause of this phenomenon has 

 been shown by the absence of prolonged homograft acceptance 

 if genetically related parabiotic partners are grafted with homo- 

 logous skin transplants (Nakic and Silobrcic, 1962). 



The majority of separated parabionts that failed to destroy the 

 cross-graft fell ill and died of an illness which we had named 



