Recently, increased attention is given to genetic constitution of the host animals or their environment in studies of the relationship between the organisms and the host animals. It has been found that the susceptibility may differ depending upon mouse strain. According to the results reported in our country, C57BL/6 is susceptible to tubercle bacilli (SOMEYA, S., et al., 1955), CF#1 is susceptible and CFW is resistant toS. abortus equi (FUJIWARA, K. et al., 1957), C3H is resistant toB. anthracis (TAKIZAWA, T. et al., 1958), C57BL/6 shows the most rapid formation of lepromata when injected subcutaneously with murine leprosy bacilli in contrast to how lepromata formation in C3H (KAWAGUCHI, Y. 1957) .
This paper is concerned with the response of various strains of mouse to experimental inoculations of Brucella organisms. The results were as follows:
1. The response of each strain of mouse toBr. abortuswas qualitatively the same. Therefore, the response shown by the dd strain may represent that of all strains of mouse toBr. abortus.
2. A slight quantitative difference in the response was shown from one strain to another. The CF# 1, C57BL/6, and dd strains gave a similar mortality rate from large-size inocula, while the former two strains showed a more rapid course than the dd strain. The C3H and CFW strains manifested a lower mortality rate and slower course than the dd strain. The multiplication of the organisms in the organs in an early stage was more apparent in the first three strains than the latter two. The strain C3H gave comparatively weak and rapidly progressing histopathological responses.
3. A comparatively small deviation in the response was shown with the dd strain, a general purpose strain, as well as with other specialized strains of mouse.

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The virulence of tubercl becilli to experimental animals varies widely according to the conditions of host and parasite. The study was undertaken with the purpose of analyzing the relation between the environment for the growth of tubercle bacilli (Ravenel strain) and their virulence. The following conditions were kept as constant as possible such as the inoculum dosis, the state of dispersion of bacterial cells in suspension, the age of culture, and some conditions of experimental animals (strain and age) .
I. Observations on the organismsin vitro
a) A culture in tween-albumin liquid medium (Dubos' liquid medium) induced more severe tuberculsis in mice, rabbits or guinea pigs than a culture in Sauton's synthetic medium or Ogawa's solid medium. (see Table 1, 2 and 3)
b) The difference of virulence between Dubos' cultlre and Ogawa's culture was kept unchanged through a series of passages in mouse lung. (see Table 4)
c) The virulence of Dubos' culture was lowered easily by subculture in Ogawa's medium. (see Table 5)
II. Observations on organismsin vivo
a) A strain from lung of the mouse, injected with Dubos' culture, was more virulent to mice, rabbits or guinea pigs than a strain from liver of the same mouse. (see Table 6and7)
b) The strain from lung was decreased in virulence by passage through liver of mice, on the other hand, the strain from liver was increased in virulence by passage through lung. (see Table 8)
c) The strain from lung was increased in virulence by subculture in Dubos' medium, while decreased by subculture in Ogawa's medium. Though the strain from liver was increased in virulence by subculture in Dubos' medium, this subculture did not show the same level of virulence as the subculture of the strain from lung in Dubos' medium. No difference in virulence was observed between the strain from liver and subculture of the same strain in Ogawa's medium. (see Table 9)

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