Since the epidemic outbreaks of encephalitis in St. Louis and Kansas City, U.S.A. and in Tokyo, Japan, numerous research works have been reported by American and Japanese investigators on the isolation of causative virus or on its immunological and epidemiological studies.
However, those questions relative to the route of infection or the affinity of the causative virus to various organs have not yet been explained experimentally in a satisfactory manner. The above is considered due to the facts that the epidemic is relatively sporadic, that family infection is quite rare and that there are a number of inapparent infections with serologically positive findings. Not all of the individuals infected will develop the symptoms, thus necessitating one to consider the factors inhibiting actual occurrence of the disease on both virus and host sides. The mosquito transmission theory of encephalitis virus presented by T. Mitamura et al and W. McD. Hammon & W. C. Reeves et al has gained general acceptance nowadays as the most reliable theory, through a number of researches made both in laboratories and in the field. But even this theory has a weak point in explaining the reservoirs of encephalitis virus present in nature. The same remarks can be made on the nasal instillation theory, which was presented by R. Kawamura et al. Process and mechanism of the infection exercised by the virus and the host, therefore, are the most essential aspecLs for evaluating such theory. Such a study, however, is extremely difficult, because of the involvement of too many factors which need a considerable length of time to dissolve. But as the first step into such a field, experiments were carried out to know the affinity of virus to various organs of experimentally infected mice. Numerous experiments have already been reported on this subject but those made on Japanese B encephalitis virus in comparison with several other strains of similar nature are extremely rare. There are two principal attitudes to this problem. Some investigators are inclined to explain it by the multiplication of virus in organs, except central nervous system, in the initial days, and others are against them.
K. Ando and his co-workers have isolated two strains of encephalitis virus, Negishi and K-13, from the patients occurred in Tokyo during the epidemic of Japanese B encephalitis in the summer of 1948. Serological differentiation of these two strains was not practicable. Comparing the affinity of Negishi strain to various organs in mice with that of Nakayama (Japanese B encephalitis virus) and St. Louis encephalitis virus, Negishi strain was found to infect mice by oral administration more easily than Japanese B encephalitis virus or St. Louis encephalitis virus. Comparisons of biological characters, especially their affinity to blood and central nervous system, were made with Russian spring summer encephalitis virus (western type) . At the same time, comparisons were made with Nakayama and St. Louis encephalitis viruses, which have been passed through many brains of mouse, and also with those viruses which have not been passed through so many mouse brains.
The experiments described above are one of the measures to clarify the course of infection on the basis of affinity to various organs of mice, which is not yet determined to lie whether on the side of virus or host.
Reported herein are the results obtained from those experiments described above. The results were not of full satisfaction, but some biological differences existing among encephalitis viruses were clarified by them.

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There is no specific treatment for rabies once the disease has developed. Rabies is a peculiar disease, however, because of its known exposure to the infection and of the possible protective treatment during its incubation period. At present, the prophylactic treatment after exposure is mainly performed by the use of various types of vaccine, taking the advantage of its longer incubation period. But, this treatment is not always perfect to prevent development of the disease, especially when bitten on the face by a rabid animal. Further, postvaccinal paralysis occurs in a few cases. Improvement of the vaccine and some other antirabic treatment should be studied to eliminate these defects. Fe-investigation of the effect of antirabies hyperimmune sera including the use of concentrated and fractionated sera will be of some value, as it is practicable, when human beings are exposed to the infection, to initiate the treatment in relatively earlier stage of its incubation peried.
The studies on protective effects of rabies antiserum have been performed by many workers since many years ago. As the historical summaries of this problem are found in many papers by these authors, it is considered unnecessary to describe them in details. The opinion, such as concluded by Marie, that the infected animals cannot be protected by injections of the antirabies sera, was accepted by many workers in old times, although Fermi (1) has reported on the effective antiserum treatment. Thereafter, Proca et al, Hoyt et al, and Yaoi et a1 reported on this problem, and recognized, though varying in grades, the effectiveness of antirabies serum.
Recently, Habel performed detailed experiments on antiserum treatment in which monkeys, guinea pigs and mice were used. He reported that antiserum treatment was more effective than vaccine treatment and that the infected mice were protected when antiserum was used within 3 days after the virus inoculation. Koprowski et al, furthermore, treated guinea pigs and hamsters with the antiserum fraction prepared by dialysis and also described about the application on infected human cases.
Thus, recently many reports have been made recognizing the protective value of the antiserum treatment against rabies infection, though generally they were not accepted according to the papers of old times. Descriptions on virulence of the inoculated virus and antibody titers of the antirabies serum used, however, were incomplete in the majority of those reports. Particular attention was paid on this point in the present paper.
The report herewith is the results obtained from the antiserum treatment after the infection with fixed virus. Experiments on serum treatment after the infection with street virus are undergoing at present and a separate report will be made later on.

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