An acute "coughing disease" broke ont in the greater part of the racehorses and riding ones belonging to the Equestrian Park, the Fukushima, Tokyo, and Nakayama Race Courses in December, 1971. The epzootic seemed to have been spread quickly by imported horses which might have been infected with the "coughing disease". In this outbreak infected horses showed the same clinical symptoms, such as fever at 39-41°C, dry cough characterized by strength. Equine influenza viruses were studied to be isolated from 20 of the samples collected from the masopharngeal secretion of febrile horses, inoculated into the amnion of 8 day chick embryos and cnltivated at 35°C for 72 hours. They have been adapted to grow in the chorio allantois of chick embryos. The hemagglutination-inhibition (HI) and neutralization tests were performed with chicken and ferret antisera against A/Equi 1/Prague/56, A/Equi 2/Miami/63, and the isolated (Tokyo/ 71) viruses. The intact viruses were used as antigens. Furthermore, when the antibody response of convalescent horses was compared between Equi 2 and the isolated virus, it was found that the antigenicity of the isolated virus was about 8-16 times higher than that of Equi 2/Miami/63 virus. On the other hand, there was not so marked a difference in HI titer between the antigens of Equi 2/Miami/63 and the isolated viruses for chicken and ferret antisera against the Equi 2/Miami/63 strain. In this result 9 strains of equine influenza virus were isolated. It is for the first time in Japan that equine influenza virus was isolated. It was confirmed that the isolated viruses were similar to the Equi 2 type, although they were distingushed from the Equi 1 type.
