MODIFICATION OF RUNT DISEASE 377 



interest that something of this kind actually does occur since the 

 graft-against-host reaction is not merely delayed but is held in 

 permanent abeyance. 



Whatever may be the mechanism of this intriguing pheno- 

 menon it now seems apparent that "tolerance" can be induced in 

 a population of lymphoid cells from an adult donor to histo- 

 compatibility antigens of animals of considerable genetic diversity 

 and that this process can be promoted by amethopterin treatment. 



Acknowledgements 



This work was supported by grants A-4497 and GSF-5110-C1-B from the 

 United States PubHc Health Service. I should hke to express special appreciation 

 to Miss Mary L. Wood for her expert technical assistance. Thanks are also due 

 to Mr. Eric Grave for assistance with the photographs and to Miss Margaret 

 Wilkie for help with the manuscript. 



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