A number of workers have shown that antibodies to the influenza A viruses which are demonstrable in the sera of normal adults undergo periodic fluctuations in titre related to epidemic occurrence of the disease. The rise in antibodies accompanying infection and demonstrable by neutralization, agglutination-inhibition and complement-fixation tests is used as a method of diagnosis which is admitted to be of greater sensitivity than actual recovery of virus from the throat. Studies of the population during an epidemic have also revealed the existence of subclinical infection with rise in antibodies comparable to that occurring in those suffering clinical illnesses. But surveys of sera from large samples of the population show no general upward shift in antibody levels unless an actual epidemic occurs (Martin, 1940). The studies made by Francis, Magill, Rickard & Beck (1937), Hoyle & Fairbrother (1937), Rickard, Lennette & Horsfall (1940) and Martin (1940) also indicate the relative impermanence of the enhanced antibody levels consequent upon an epidemic. A relatively rapid decrease in antibody is shown both by the neutralization and complement-fixation tests during the 3–6 months after an epidemic and then a slower fall occurs until the next epidemic again causes a rise in titres. There have, however, been relatively few studies on large samples of populations situated in different geographical areas before and during an outbreak.