At the end of March 1956, an influenza epidemic occurred in Gunma Prefecture, and by virus isolation and serological tests it was identified as one due to the type A.
In the course of this epidemic 7 strains were isolated in this district, and the hemagglutination inhibition test with RDE-treated immune sera indicated a close antigenic relation of these strains to the current strain of influenza A. Antigenic structure of these strains were similar to that of Ashiya 1-55 strain or Omachi 1-53 strain but. different from that of PR 8.
The virus was indistinguishable from the hitherto described influenza A virus in the thermo-stability of its hemagglutinin and in the rate of elution from erythrocytes.
From 11 patients the serum was obtained either paired at the acute and convalescent stages or at the latter stage alone and submitted to hemagglutination inhibition test. with 5 prototype strains of influenza and HVJ viruses.
Almost all patients exhibited a significant rise of antibody to the current strain of influenza A in the convalescent phase.
RDE prepared from the culture filtrate of V. cholerae and employed in the abovementioned test, contributed much to yield clear-cut results, because it eliminated nonspecific inhibitor in the antisera without imparing specific antibodies.