In January, 1953, there was an outbreak of swine influenza-like disease in Shizuoka Prefecture, and in July, 1953, another outbreak of the same disease in our institute. In our laboratory, the virus was isolated from three materials of lungs with haemorrhagic lesions and consolidation by conducting intranasal inoculation into mice, and from another two materials by practicing amniotic method. As regards the elevation of antibody in a swine serum against the virus, the complement-fixation reaction and the haemagglutination-inhibition test were proved effective. Our finding of the swine inoculated this virus is as follows: Observed were pathologically, oedem and haemorrhage of lung, follicle swelling of spleen, haemorrhage of urine bladder, swelling and congestion of each lymphnodes, and chinically, slight rise of temperature, (40-50.5°C), chilliness, decrease of appetite and coughing. Beside this, what were observed in circulating blood are such as the appearance of myelocyte and metamyelocyte, and the increase of rod nuclear cells in neutrophiles and of monocyte. As a result of making tests in vitro and vivo on the newly isolated virus, we learned of its main properties as follows: 1. All the mice inoculated the virus through intranasal route, died after 4 to 8 days, presenting such a symptom as consolidation of lung, similar to that of influenza. Microscopically observed was catarrharic broach-pneumonia with severe infiltration of neutrophile leucocytes and necrosis of bronchial epithelcell. Virus multiplication was observed in spleen, lung and lymphnode. 2. As to the isolation of the virus from the swine, the amniotic method was successful, while the allantoic method was not. In case the virus was demonstrated by haemagglutination in the second or third serial amniotic passage, the cultivation of the virus in allantoic cavity and yolk sack of embryo could be made possible and the serial passage of the virus, made easy to develop through any route. 3. When the virus was inoculated into yolk sack of 6 to 8 day old embryo, the embryo, as a rule, was killed 48 hours after the inoculation; but it survived when the virus was inoculated on chorioallantoic membrane or in allantoic cavity. Regardless of its route, the virus developed positively in 48 to 96 hours after the inoculation. 4. The virus titer in allantoic fluid usually indicated 10-5 to 10-7 and haemagglutination titer, 1: 1, 000-1:2, 000. And the haemagglutinative potency of the virus was demonstrated in the red cells of fowl, guinea-pig, sheep, goat, pig, mouse, pigeon, human (o type), rabbit and cattle, but it turned out negative in the red cell of horse. 5. The virus was completely adsorbed into fowl red cell at 4°C in 30 minutes and was eluted at 22°C in 10 hours. 6. The haemagglutinin was destroyed by heating at 50°C in 5 minutes, while the infectivity was entirely destroyed at 22°C in 30 days, and by heating at 50°C in 10 minutes. 7. In the process of isolation of the swine virus, the variation of phases “O” to “D”, which was described by Dr. Burnet et al. in the process of isolation of human influenza A virus, was observed. Outline of the variation of strain S was schematically shown in Chart No. 8. Also, in the case of isolation of strain 153, HME and 3471, the similar phenomenon was observed. 8. The filtrability of the virus, going through Seitz EK, Berkefeld w. and Chamberland L3, was proved.