PROTECTION AGAINST HUNTING 291 



and tha wings (-70° to +37° c) were rapidly performed in 

 succession and the material ground i min. with an Ultraturrax 

 grinder and subsequently submitted to 10 more freezings and 

 tha wings. After lyophilization, the vacuum-frozen-dried material 

 was stored (at - 20° as a further precaution) in separate ampoules 

 corresponding to 4 ml. of the original suspension. Just before use, 

 the lyophilized powder was diluted with the corresponding 

 volume of distilled water. 



(b) Spontaneous tumours indigenous to strain A were also used as 

 immunizing material. The one which was mainly used was a 

 spontaneous abdomino-thoracic tumour, the nature of which 

 unfortunately remained unknown. 



(2) Sera presumed to contain enhancing antibodies 



These sera were of two types and will be referred to as AN-BI, 

 II or III (anti-A-newborns) and AN-B+T (anti-A-newborns 

 and anti-A-tumours). 



(3) Homologous living cells injected into newborn mice 



These cells were always CBA spleen cells. Adult CBA mice 

 were splenectomized and the splenic pulp was gently pressed out 

 of the capsule into a Petri dish containing physiological saline. A 

 homogeneous suspension was obtained by projecting the cells 

 repeatedly with a Pasteur pipette on the wall of the dish. The 

 suspension was then spun down in a refrigerated centrifuge 

 ( + 4°) at a speed of 800 rev./min. for 10 min. The supernatant 

 was discarded and the cells resuspended in an adequate volume of 

 heparinized saline adjusted to a concentration of 120-160 million 

 cells per ml. of suspension. Larger particles settled out by sedi- 

 mentation within 3 min. 



C. Materials to test the sera 



The dextran solution was Intradex, kindly suppHed by Glaxo 

 Laboratories. Other materials were as described under Methods. 



