310 GUY A. VOISIN AND RADSLAV KINSKY 



cent (± 5 per cent) of the litters, while the reverse situation occurred 

 in 21 per cent (± 4 per cent) of the litters (for a typical litter, see 

 Fig. 2). It can be concluded that the observed differences, although 

 small, are probably significant. Therefore, it can be said with 

 some confidence that there is a significant decrease in the mean 

 weights of experimental animals as compared to normal controls, 

 especially after day 7. Furthermore, there is a shght but probably 

 significant decrease in the mean weights of unprotected animals 

 as compared to protected animals. This difference is already 

 noticed (although extremely small: o* 10 g.) on day 7. This effect 

 on weight occurs in addition to the difference in death rate since, 

 obviously, only the hving animals are weighed. The two facts 

 that there is a difference in weight and that this difference is a very 

 small one seem to be of importance. 



That such a protection against runting may be obtained by 

 doses of "facilitating" serum as small as 0-20 mm.^ can seem 

 somewhat disturbing. At any rate, it raises the question of the 

 specificity of this action. The following section answers this 

 question. 



II. Immunological specificity of the sera 



Immunological activity and specificity of the sera were 

 tested both in vitro by haemagglutination and in vivo forimmuno- 

 toxicity toward A newborn animals. 



A. Haemagglutination 



Haemagglutination of A red cells — performed according to 

 Gorer's technique, slightly modified (see Methods) — by anti-A 

 sera gave the following results: serum from CBA mice injected 

 only with lyophilized strain-A newborns (sera AN-BI, AN-BII 

 and AN-BIII) gave negative results, while serum from CBA mice 

 injected with lyopliilized strain-A newborns and grafted later 

 with A tumour gave positive results, graded as 64 (+ + + 16) or 

 65 (+ + + 16-32) (see Methods for an explanation of these 



