cH.viii AGGLUTINATION OF B. PESTIS 201 



showed that sterilised cultures (solid growth on gelatine 

 and agar broth cultures) repeatedly injected, in large 

 doses, subcutaneously or intraperitoneally, into guinea- 

 pigs, do not confer absolute protection on the guinea-pigs 

 against further infection any more than does repeated 

 injection of sub-fatal doses of living plague bacilli. In 

 fact, I showed in an unmistakable manner that the guinea- 

 pig is an animal which it is extremely difficult to immunise 

 against plague. This fact was subsequently verified in a 

 series of experiments which Professor HafFkine and I 

 together carried out in my laboratory with his plague 

 prophylactic. We found that even injection of enor- 

 mous doses of Haffkine's plague prophylactic, such as 

 yielded positive results in protection of rats, did not confer 

 absolute protection on the guinea-pigs against subsequent 

 infection. 



In the same report I have also described experiments 

 which conclusively show that, as was to be anticipated 

 from the above negative results, the blood of guinea-pigs 

 which had recovered from induced plague (produced by 

 injection of sub -fatal doses of living plague bacilli) 

 is devoid of immunising or germicidal substances in 

 appreciable amounts. I have in further experiments 

 sought to ascertain whether the blood of guinea-pigs 

 previously prepared, either by repeated injection of sub- 

 fatal doses of living cultures or by repeated injection of 

 large doses of sterile cultures, possesses any agglutinating 

 action on emulsion of plague bacilli ; and it is these 

 experiments which I propose here to describe. 



It is now well established that by repeated injections 

 of an animal with a particular microbe the blood and 

 tissues of this prepared animal undergo certain changes 



