332 B. NAKIC, A. KASTELAN AND N. AVDALOVIC 



in 45 per cent of recipients. The Y59 strain is being developed 

 from the Y stock used in our previous experiments. The results 

 of some recently conducted grafting tests showed that grafts on 

 over 80 per cent of hosts were still in good condition after 100 

 days. 



The technique of skin grafting and the parabiosis operation 

 have been described elsewhere (Nakic et al., 1961; Nakic and 

 Silobrcic, 1962). The normal mean survival time (± standard 

 deviations) of Wistar grafts in Y59 recipients is 8-4± i-i days 



Table I 



Survival times of homografts exchanged between Y59 

 AND Wistar parabionts* 



8, 2X 9, 3 X 10, 3 X II, 2X 12, 

 Y59 i 3 X 13, 2X 14, 16, 21, 23, 28, 33, }■ > 34 



47, 48, 65, 73, 78, 4X >ioo 



* Operation for parabiosis and cross-grafting performed simultaneously; parabionts 

 were separated after six days. 



t Normal average survival times (+ standard deviations) of skin grafts transplanted 

 from Y59 to Wistar rats and vice versa are as follows: 



Y59—> Wistar 9-6+1 -2 days 



Wistar->Y5 9 8 • 4 ± i • i days 



and that of Y59 grafts in Wistar hosts 9 • 6 ± i * 2 days. Adult 

 rats of both sexes were used. Parabionts were separated after 

 six days. The results are presented in Table I. 



As can be seen, prolonged survival of grafts was induced in 

 parabionts from both strains, but the degree of tolerance conferred 

 upon Wistar parabionts was very low, the breakdown of all 

 grafts having been completed by the 13 th postoperative day. 

 Such asymmetry in the tolerance responsiveness between Wistar 

 and Y59 parabionts was not noted in the Y59->W combination* 



* The W strain is genetically unrelated to Wistar rats and strains derived 

 therefrom. 



