Nagaoka City, a city with a population of about 150, 000 in Niigata prefecture, experienced an epidemic of poliovirus type 2 infections in 1958 in which 21 paralytic cases were observed, showing a case incidence of approximately 40 per 100, 000 population (Saito et al., 1960, Kitaoka, 1960) . The Health Authorities of the city had been interested in introducing protective measures against polio infections. On the other hand, live oral poliovirus vaccine had been studied more and more widely (First and Second International Conference on Live Poliovirus Vaccine, 1959 & 1960) and in 1959 one of the authors (M. K.) received Sabin's original vaccine from Dr. A. B. Sabin for field trial in Japan. The first field trial of Sabin vaccine in Japan was thus undertaken in Nagaoka City in 1960. The following is the outline and results of the trial.

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Among various problems developing during the epidemiological and virological surveys after the mass administration of live oral poliovaccine, the identification of the origin of the polioviruses isolated from vaccinees and contacts, especially from clinical specimens of suspected poliomyelitis patients, within a short period (one to two months) after vaccination is one of the most important problems. Genetic markers such as d, rct/40 and monkey neurovirulence have been found rather unstable, when the vaccine virus undergoes multicycle proliferation and passages through the human intestine, while the serological properties have been considered more stable and strain-specific under such circumstances. The inf ormations useful for the intratypic serological differentiation of the strains of poliovirus have been reported by several workers (Wenner et al., 1959; McBride, 1959; Gard et at., 1960; Wecker, 1960; Plotkin et at., 1961; Nakano and Gelf and, 1962, 1963; Wasserman and Fox, 1962; Vonka et at., 1962; Woods et at., 1962; Ozaki et at., 1963; Kitahara et at., 1963_a) .
During the study on the stabilizing effect of L-cystine on various strains of poliovirus against thermal inactivation, it was found that there were considerable differences in the rate of the thermal stabilization by cystine between the strains derived from the attenuated poliovaccine strain (Sabin) and other wild or virulent strains, which belonged to the same immunogenic type. This communication deals with the kinetics of the thermal inactivation of various strains of poliovirus in the presence of different concentrations of L-cystine, and the results obtained by a simplified method for intratypic differentiation of poliovirus by the use of cystine.

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Tagaya et al. (1961) isolated an attenuated mutant strain of dermovaccinia virus during serial one-day-egg passages of a dermovaccinia strain, DIE. The mutant strain, DIs, showed almost no pathogenicity for experimental animals except chick embryos, whereas it was serologically identical to the original strain. The mutant virus couldd not grow luxuriantly in the culture of HeLa, FL or L cells. Primary cultures of monkey kidney and rabbit kidney cells could not support the full growth of the mutant, . either.
In the experiments with primary cultures of chick embryo cells it was found that the strain DIs could grow as well as the original strain DIE and that the inoculation of DIs virus, either intact or ultraviolet (UV) -inactivated, could interfere with the multiplication of DIE virus. Further experimental data suggested that the phenomena could be accounted for by the production of interferon by DIs virus.

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