Inhibitory effect of formycin and formycin B on the intracellular multiplication of goat pneumonitis agent (GP), a member of Chlamydia psittaci, was investigated. Drug was added to the nutrient fluid at two hours after inoculation to monolayers of either Earle's L or Yasumura's VERO cell lines. Infective units produced in cells cultivated in media which containd the antibiotic at varying dosage were assayed after an incubation period for 48 hours at 37°C. The “inclusion forming units” were assayed by the “infected cell count method”.
The inhibitory action of both formycin and formycin B on GP-agent was far more effective in VERO cells than in L cells. The growth of the psittacosis agent in VERO cells was completely inhibited by formycin at a level of 1.4mcg per ml, whereas the drug did not inhibit the multiplication of the agent at the same concentration when L cells were chosen. The result may indicate that the psittacosis agent itself is resistant to these drugs, and that the inhibitory action is a result of some inhibitory effects of the host cell metabolism.
The inhibitory effect of formycin and formycin B on the psittacosis agent can be reversed to some extent by an addition of excessive purine or purine nucleoside to the nutrient media. Inosine, Na-inosinate, adenine or adenosine reversed the inhibitory effect of the two antibiotics. However, guanine and guanosine did not reverse the inhibitory effect. This may suggest that these drugs act as purine analogues, inhibiting certain essential metabolic pathway where adenosine may act an important role.

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A method for titrating the infectivity of a Flacherie virus of the silkworm has been described, with special interest to determine the minimum number of virus particles required to cause infection in a larva of the silkworm. Partially purified virus particles were serially diluted and inoculated per os to newly hatched larvae of the silkworm (of a Japanese race H4×H4). Most larvae inoculated died between 6 and 12 days after the virus inoculation. Contamination experiment showed that sick larvae due to contamination with feces of the inoculated larvae died always later than the 13th day after the inoculation. Therefore, virus titer (LD₅₀) has been calculated based on the mortality ratios determined by the 12th day after the inoculation. The obtained mortality ratios of the virus-infected larvae groups bore a close resemblance to the theoretical curve (derived from Poisson's formula) on the assumption that a single virus particle is sufficient to initiate infection in a larva.

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Serological relationship among several strains of the Flacherie virus of the silkworm isolated from various areas in Japan (Nagano, Saitama, Hyogo, and Shimane Prefectures) has been determined by means of serological neutralization technique. Strong cross neutralization has been demonstrated between any pairs of virus strains and anti-virus serum, so far as tested. It is concluded that the Flacherie virus group of the silkworm in Japan may be consisted of serologically related strains.
The protective effect of the anti-Flacherie virus rabbit serum against per os virus infection to the larvae of the silkworm has been demonstrated. When the anti-Flacherie virus rabbit serum was ingested by the newly hatched larvae just before per os inoculation of the virus (at 1, 000 infectious units per one larva) all larvae survied, whereas all larvae that had not been treated with the antiserum died by the 10th day after the virus inoculation.

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Hemagglutination Inhibition (HI) and Virus Neutralization (VN) test are widely used as antibody detection methods in Newcastle disease. The evaluation of the tests as an index of immune status has not yet been established, particularly HI test. The present paper deals with the correlation between antibody and protection against challenge and the susceptibility of antibody against 2-mercaptoethanol (2-ME).
The correlation was obtained between the VN titer and the protection rate in chicks at 4 days of age, vaccinated with B₁ strain of the virus and challenged 2 or 5 weeks after vaccination. The similar relationship was observed between HI titer and the protection rate, when challenged 5 weeks, but not, when challenged 2 weeks.
The ratios of HI titer to VN titer were different at the stage of immunization; the ratio was smaller in sera collected 2 weeks after vaccination than that of 5 week ones. This findings suggested that the qualitative change of antibody took place.
Then the 2-mercaptoethanol susceptibility of antibody was tested. The antibody sensitive to 2-ME appeared in serum during the first to second week after vaccination and was replaced with the resistant one in the following weeks.
The appearance of the 2-ME sensitive HI antibody seems to be a reason why the HI titer did not correlated to the protection rate in 2-week-challenge case.

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It was previously suggested that the character of the A-type inclusion can be separated into two markers: the capability to include mature progeny virions within or on the inclusion body (V), and the formation of ground substance of the inclusion body (In). In addition, another difference in the susceptibility to the L cells was found between the vaccinia IHD strain virus and the CPR-Cl cowpox virus (L). The low growth rate of CPR-Cl virus calculated about 3 logs can be rescued by vaccinia IHD virus in mixedly infected L cells.
The recombinants among these markers were found in variable percentages of the progeny viruses of mixed infection with a combination of vaccinia IHD and cowpox CPR-Cl viruses, using the plaque types as control markers. Especially our prediction that the phenotypes of the A-type inclusion is controled by two viral genes was proved by the appearance of V⁺In⁺ recombinants of 1.8%. Concerning the plaque types being of polygenic nature, morphological features of cytopathic effect in HeLa cells were coincided with plaque types of all recombinant clones.

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