While the theory that a filtrable virus is the etiologic agent of epidemic influenza had been repeatedly expressed, no convincing evidence had been adduced to support the thesis until the recent studies initiated by the observations of Smith, Andrewes and Laidlaw.1These investigators reported in 1933 that ferrets were susceptible to infection with a filtrable virus present in the nasopharyngeal washings of patients suffering from epidemic influenza. Since that time strains of the same virus have been isolated in different parts of the world, and in each instance the recovery of virus has been associated with an epidemic of influenza. In addition, it was found that antibodies capable of neutralizing the infectious agent were present in the serum of human beings, most marked in thosewho had recovered from recent attacks of the disease. Furthermore, it was our opportunity to observe in 1934 the actual development of antibodies to the