STUDIES ON THE SERUM OF VACCINATED ANIMALS. 69 



alone produces Pfeiffer's phenomenon without the aid of normal 

 serum. Tubes of fresh serum from well-immunized guinea-pigs 

 were heated to 50 degrees, 55 degrees, 60 degrees, and 64 degrees 

 for 5 minutes. Each one of these sera was then mixed with an 

 emulsion of cholera. 



As may be seen from this table, serum heated to 50 degrees trans- 

 forms the vibrio; heated above this temperature it has no effect. 

 The bactericidal substance in the serum of immunized animals, 

 then, is destroyed at the same temperature as in normal animals. 

 But this heated serum, which alone cannot produce Pfeiffer's phe- 

 nomenon, brings it about perfectly well, and apparently with undi- 

 minished vigor, when normal serum is added. In other words, 

 normal serum restores to the immune serum the property lost by 

 heat. A mixture of two fluids, each of which alone has no bacteri- 

 cidal property, forms a fluid that has high bactericidal property for 

 a specific organism. We have already described this experiment 

 and in the previous description we demonstrated the regeneration 

 of bactericidal power, not by means of Pfeiffer's phenomenon, but 

 by gelatin plate cultures. 



What conclusions may be drawn from these facts? We have 

 already considered them partially and these experiments only 

 confirm them. It would seem as if the serum of vaccinated animals 

 had no 'particular bactericidal substance, but that a similar bactericidal 

 substance is present in the blood of normal as well as of immunized 

 animals. This bactericidal substance is not specific unless mixed 

 with* the preventive substance, and under its normal conditions will 

 affect only attentuated vibrios. Its energetic action depends on 

 the combined presence of a preventive substance that is present only in 



