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immune serum; all proved to be immune upon reinoculation with the 
virulent pest bacillus. The organism with which these vaccination ex- 
periments were performed possessed so little virulence that from two to 
three living, agar slant cultures, when injected into a guinea pig of 250 
grams’ weight, did not cause the death of the animal. In the experi- 
ments of Kolle and Otto on the vaccination of monkeys, almost all of 
the animals died from the effect of the attenuated culture or succumbed 
to intercurrent disease. 
Other experiments on guinea pigs, in which repeated inoculations of 
the killed cultures of the plague organism were employed, were also per- 
formed. The animals were first injected with one, then with 1.5, and 
finally with two killed agar cultures, or with 1, 1.5, and 3 cubic centi- 
meters of Haffkine’s prophylactic. In the process of immunizing twenty 
guinea pigs, six of the animals died from the effects of such large doses 
of the killed bacteria. ‘The immunity of the remaining fourteen, six 
weeks after the last injection, was tested with the living virulent plague 
bacillus, when only one animal remained alive and proved to be immune. 
Therefore, Kolle emphasizes the fact that, if such large and repeated 
doses of the killed pest organism fail to immunize such small animals as 
guinea pigs, it seems unreasonable, from such a method, to expect very 
favorable results in man, particularly since in the latter case the amount 
of the bacteria inoculated is so much smaller in proportion to the body 
weight. It is to be noted that in immunizing the guinea pigs similar or 
larger doses of killed organism were employed than have been’ recom- 
mended for protective inoculation in human beings. 
It is not my purpose here further to enter into the discussion of the 
protective value of the different prophylactics recommended for the 
immunization of man against plague. I had already concluded from 
animal experiments, as well as from the fact that a number of persons 
who had received several injections of Haffkine’s prophylactic later sick- 
ened and died with plague that the ki/led pest organism constituted for 
man a far from satisfactory protective against this disease. On the other 
hand, the experiments which Kolle and his pupils had performed on 
guinea pigs seemed so conclusive in regard to the value of the living 
attenuated cultures in the immunization of these animals that I felt 
convinced that, if these cultures were of so low a virulence, or could be 
further so attenuated as to warrant their use in man, a higher degree of 
immunity could almost certainly be obtained with them than by the 
employment of the killed bouillon or agar cultures. 
Accordingly, when Professor Kolle, after some correspondence on the 
subject, kindly offered me cultures of these attenuated pest organisms 
which had been employed in his experiments on guinea pigs, for use in 
the vaccination of human beings, | decided to carry on this work. My 
experiments in vaccination in man and animals have been performed with 
three attenuated strains of the pest bacillus, Maassen Alt and Maassen V 
