296 GUY A. VOISIN AND RADSLAV KINSKY 



State after 100 days were never spontaneously rejected. For that 

 reason, several tolerant animals were killed after four months for 

 other experiments. However many tolerant animals were kept 

 under observation for periods up to 13 months. 



G. Controls 



Various controls were set up at different steps of the 

 experiments. 



(i) Histocompatibility 



Histocompatibility was repeatedly tested by skin grafts, hitra- 

 strain grafts were uniformally accepted even between the CBA 

 mice from Bar Harbor and from London while inter-strain 

 grafts between A and CBA were rejected in all cases within 

 10*5 ± I days in both strain directions. 



(2) Viability oj injected spleen cells 



Viability was ascertained by high incidence of runting and 

 high percentage of tolerance. Viability tests with eosin were used 

 at the beginning and then discontinued. 



(3) Activity of antisera 



Whatever test was used (haemagglutination, toxicity or facilita- 

 tion) the immune activity of CBA sera was controlled by similar 

 dilution of normal CBA sera wliich remained invariably devoid 

 of activity. 



(4) Additional controls in the study oJ protection against runting 



(a) CBA immune sera alone were tested at various dilutions. 

 Dilutions non-toxic for the newborns were used. 



(b) As some variations were noticed in the weight curves 

 of untreated htters over different periods of the year, we kept 

 uninjected controls under observation and their evolution was 

 compared with that of experimental animals. Only the litters 

 in which no death at all was observed among these uninjected 

 controls from day 7 to day 3 5 were accepted for study. This led 



