The pellets sterilized by gamma radiation were tested for breeding of barrier-sustained rats. Sterilization was sufficiently achieved by an exposure of the pellets to 3 Mrad. The rats fed the pellets exposed to 3 Mrad showed a reasonbly reproducibility and their offsprings which were fed the same pellets grew well as compared to the animals given autoclaved pellets, although exposure of the pellets to 6 Mrad affects parturition rate and litter size significantly.

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Livers from germfree ICR mice have been examined for esterase isozymes by means of agargel electrophoresis. Variations of banding patterns in the intermediate zone of esterase isozymes were detected in germfree ICR mice, bred in vinyl isolators.
In our previous research, four different patterns of esterase isozymes in the livers were detected in 12 strains of mice, and we divided them into Type I, II, III and IV. The esterase isozyme pattern was uniform in every inbred strain, but variations of banding patterns were observed in closed-colonies. Type I esterase isozyme pattern was the most common in the mice we investigated, and in this type, about 17 esterase bands were observed. Esterase isozymes of germfree ICR mice belonged to Type I, however, two subtypes (I-a, I-b) were found. Most of the germfree ICR mice exhibited the I-a pattern of esterase isozymes which consisted of adout 17 bands, but several mice exhibited the I-b pattern in which modification was observed in the intermediate . zone of zymogram. (For the patterns refer to Fig. 1) In isolators No. 10-b and 13, there were very high percentages of Type I-b, while the remainig isolators had no Type I-b, and mice with the I-b pattern originated from one pair of parental mice (Table 1 and Fig. 1) Type I-b was also present in 14.3% of closed-colony mice.
It is thought that the variations in closed-colony mice are caused by genegtic segregation and the variation in germfree mice are due to unavoidable sib-mating in the isolators which results in the occurence of subdivision. Our results suggest that genetic subdivision occurs in germfree mice bred in vinyl isolators, if they originated from closed-colonies.

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The results until the 10th generation were reported on the selection for mammary tumorigenesis of mice and on the changes in mammary development and reproductive ability accompanying with selection.
Swiss albino mice used had been maintained by random mating until brother-to-sister mating was started in May, 1964 by the present authors. They were mated at 70-90 days of age and concurrent pregnancy was planned until the 6 th lactation or mammary tumor appearance. The litter size was reduced to 8 only when it was more than 9 and the other adjustment in litter size was not done.
Individual selection was performed towards two directions by tumor age (a high and a low mammary tumor groups ; groups H and L, respectively) . The average tumor age and tumor incidence of the family were also taken into consideration for selection. The mice were checked for palpable mammary tumors once a week and all of the tumors were diagnosed as adenocarcinoma by histological observation. The reproductive histories and tumor size were recorded individually.
The tumor incidences in breeding females of H group were approximately 90% in most generations and were always higher than those of L group which were decreased in successive generations (Fig. 1-A) . The mammary tumor age in the breeding females of H group became smaller with generations, while the changes of the tumor age in L group were not so marked and were always higher than those of H group except the 1st and 2nd generations (Fig. 1-B) . The difference was observed more distinctly in tumorigenesis of virgins, especially in the tumor incidence of L group and in the tumor age of H group (Fig. 2-A, B) .
Among the items examined as the indices of reproductive ability, influences of selection were apparently observed on litter size and young's growth rate on day 12. These values were higher in H group than L group in each generation (Fig. 3-C, F) .
The mammary lobulo-alveolar formations in intact female mice and in ovariectomized mice treated with estrogen plus progesterone were much more marked in H group than in L group in almost all of the generations (Tables 1 and 2) .
The present results would be of much interest, because most of the hormonal factors controlling the mammary tumorigenesis also play the important roles in mammary development and reproduction.

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[in Japanese]

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