Outbreaks of swine influenza were first observed in Japan in 1978. A number of influenza viruses were isolated from diseased swine. Almost all viruses isolated were swine influenza virus (Hsw 1 N1) but two viruses isolated from the nasal swabs of swine showing clinical signs of influenza in the Kanagawa prefecture were characterized antigenically as Hsw 1 N2. Analysis of swine sera showed that influenza virus Hsw 1 N2 was epidemic in the farm from which the virus had been isolated. The new virus (Hsw 1 N2) seems to have been produced by recombination between swine influenza virus (Hsw 1 N1) and Hong Kong influenza virus (H3N2).