32 



It is evident that, while both the Types I and II sera were 

 highly protective for mice against the homologous strains, 

 Type I serum exercised no protective power against Type 11, 

 nor Type II serum against Type I. 



In another experiment, designed to determine whether the 

 simultaneous inoculation of protective serum and living 

 homologous culture would raise the animal's resistance against 

 a heterologous type, the result was negative : the animals died 

 like the untreated controls. 



The details of the experiment are as follows : — A mixture was made of 

 Type I serum and culture, 0*15 c.c. of serum to 0*01 c.c. of culture for a 

 dose, and was inoculated into the peritoneal cavity of mice : the serum 

 was sufficient to protect a mouse against 10 times the amount of culture. 

 The next day each mouse which had received the mixture was injected 

 with small doses, 0*000 1 and 0-000 01 c.c., of Type II culture. All the 

 mice died of pneumococcal septicaemia. An exactly similar experiment 

 with identical results was performed with Type II serum and culture, 

 followed the next day by small test doses of Type I. 



In a further experiment, mice were immunised by a pre- 

 liminary dose of Type I serum, which was followed by small doses 

 of a mixture of Types I and II culture. All the mice succumbed. 

 The result was the same when the mice received an immunising 

 dose of Type II serum. 



Each of these three experimental tests indicates clearly that 

 the protective action of such immune sera is very strictly con- 

 fined to the homologous types of infection. Administration of a 

 Type I scrum has not the least inhibitory effect on an infection 

 with Type U, and vice-versa. The same I have found to be true 

 of Type III serum as regards the other two types, I and II. 



Agglutination of Group IV Strains. 



The various races of pneumococci resemble each other so 

 closely in appearance of colonies and in the characteristic of 

 bile solubility that there can be no doubt that they belong to one 



