An isolation facility for the production and maintenance of specific pathogen free pigs was designed at National Institute of Animal Health, Kodaira, Tokyo. A total 82 piglets in 9 experi-ments obtained by hysterectomy was transfer-ed into the air conditioning room, and they were raised and maintained by technics designed to protect them from microbial contamination, i, e, all supplies were sterilized before use, and personnel put on a special designed sterile clothes. These isolated piglets were examined for bacte-rial contamination for 42 days.
1. Escherichia coli, Enterobacter cloacae, bacilli, micrococci, and pseudomonas were frequ-ently recovered from the feces of the isolated piglets during the 4 th day of operation. How-ever, all these bacterial genera except Escheri-chia coli, subsequently disappeared. Later Streptococcus faecalis and bacteroides were also frequently isolated (Table 2) . The mean bacteri-al counts per gram of the feces of the isolated piglets at 42 nd day after operation were as follows: Escherichia coli, 10^9.5; Streptococcus fae-calis, 10^8.8; Bacteroides, 10^8.4; and Proteus vulgaris, 10^6.1 (Fig.1) .
2. Death with bacteremia occured most often in the isolated piglets between 3 rd to 5 th day after operation. Escherichia coli, Enterobacter cloacae, or Klebsiella, in profuse and apparently pure culture, isolated from the blood or organs of the dead piglets (Table 3) .

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In order to control genetically the population of mice maintained as a closed colony, it is necessary to apply the theories of population genetics. Effective population size is one of the most important conceptions in population genetics. In 1938 and later, Wright derived some formula to estimate the effective size of wild animal or plant populations, and in 1957, Nozawa derived a general for.oula shown here as formula (1) and transformed it into a formula for estimation of she effective size of farm animal populations.
The author will now attempt to transform formula (1) into a formula for estimation of the effective size of a closed mice colony. If each male is mated with the same number (C) of female, as is usual in closed colonies of mice ; therefore,
Nf=CNm
kf=km/C
sm=sr=s
If the intra-class correlation coefficient (ρ) is almost zero, as shown in Table 1, and the variance (σ2km) of progenies which are contributed to the next generation by male parents is C times that of the variance (σ2kf) of progenies contributed by female parents, formula (1) can be transformed into formula (2) . In this formula, Nm is the number of male parents, C is the ratio of the number of female parents to the number of male parents, s is the ratio of the number of the breeding members in a generation to that in the next generation, and σ2km is the variance of the number of progenies contributed by the male parents to the next generation. If each male is mated with a female, namely, C=1, formula becomes formula (3), as was derived by Nozawa in 1957. When C=1 and s=1, formula (2) becomes formula (4) Which is Wright's formula.
The effective size of the ICR-JCL mice colony maintained in our Institute was estimated with formulae (1) and (2) as shown in the Table 3. Both estimations almost agree. It shows that the effective size of the ICR-JCL mice colony is between 300 and 400 in each generation, and this effective size is only about one fourth of the number of actual breeding members in this colony. The first reason for the difference between the effective size and the number of breeding members is that each male is mated with six females. The second reason is that the variances (σ2km and σ2kf) of progenies to next generation by male and female parents are larger than that of progenies contributed randomly, because the female parents contribute about five progenies per chance of contributing progenies.
The effective sizes of the population of a colony which has 50 males and various values of C, s and σ2km, are shown in Tables 3 to 7.

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Since December, 1963, a substrain of C3HeB mice has been established at the Institute for Cancer Research, Osaka University Medical School. This strain has been maintained by broth-er-sister mating and at the present, July, 1968 has reached the 20th generation. The present paper described occurrence of spontaneous tumors of this substrain ; mammary tumors (2.1%), pulmonary adenoma (3.1%), abdominal lymphoma (6.2%), ovarian cysts (2.1%) in females and hepatomas (14.3%), abdominal lymphomas (4. 0%) in males. This substrain was susceptible to the milk facter but did not pass to their offspring. The susceptibility to the agent was examined by foster-nursing on C3H/HeOs mother which carry the agent showing high incidence of mammary tumor (92.5% for breeders, 78.8% for virgins) . The foster-nursed C3HeB mice on C3H showed high incidence of mammary tumor (81.3%) but did not pass the agent efficently to their off springs with decreased incidence (42.9%) . On the other hand, foster-nursed C3H mice on C3HeB developed mammary tumors with a sig-nificantly low incidence (15.8% ), but passed the agent to the offsprings which showed high ir cidence of mammary tumors (75.0%) .

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The difference in lacta-tional performance of C3H/He and C57BL/6 strains of mice was studied by investigating weight of young, growth rate of young, milk yield and nucleic acid content in the mammary gland on day 12 of lactation. In Experiment 2, half of the litter was ex-changed between strains on the day of parturition, but not in Experiment 1. In both cases, C3H/He was higher than C57BL/6 in almost all the characters examined, in-dicating that the lactational performance of C3H/He was truly superior to that of C57BL/6.

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The use of captured wild monkeys causes much troubles in the experiments because of many known and/or unknown predisposing factors. In addition, their population in thenatural habitat seems to dectease gradually. In this note is described an outline of the breedings of cynomolgus monkeys (Macaca irus) which has been attempted in our laboratory since 1962.

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