Inoculation with Tumor Material. 
23 
at various intervals. In a number of cases the last inoculation 
gave rise to even larger tumors than in the control mice which had 
not received any preparatory inoculation. In cases in which 
mice were injected with a suspension of finely divided tumor 
material of (through heating) diminished virulence, the same 
lack of immunity was observed on subsequent inoculation with 
tumor pieces of decreased virulence. The number of mice which 
we treated with a preparatory injection of fine tumor suspension 
is however as yet relatively small and these experiments will 
therefore have to be continued. 
2. If the preparatory inoculation with material of decreased 
virulence led to a temporary tumor growth, which latter was 
followed by a spontaneous retrogression an immunity against a 
subsequent inoculation with virulent tumor material was obtained 
in many cases. This observation is in accordance with the findings 
of other investigators. This immunity is however by no means 
absolute and it will be necessary to determine in further experi- 
ments upon what factors such gradations in acquired resistance 
depend. 
3. In many cases mice in which a tumor has begun to grow after 
the first inoculation of tumor material of decreased virulence, 
can be successfully inoculated a second time with material of 
decreased virulence, and in some experiments the tumors derived 
from the second transplanted piece grow as well in mice in which 
the first preparatory inoculation had given rise to tumor growth 
as in such mice in which the first pieces had not given origin to 
tumor growth, and in such animals two tumors are growing 
simultaneously; but in the majority of cases we find that only 
one tumor grows after two successive inoculations with tumor 
material of decreased virulence. If the preparatory inoculation 
had been followed by tumor growth, the second inoculation gave 
frequently a negative result and vice versa. And especially if 
the tumor resulting from the first inoculation is growing very 
actively, a second inoculation with material of decreased virulence 
is in the large majority of cases not followed by tumor growth. 
This observation would therefore point to the conclusion that in 
a certain number of cases the growth of a tumor protects to some 
extent against a subsequent inoculation with material of de- 
