Studies were made on the rate of increase of infectivity and hemagglutinative activity of the influenzal virus B (Lee strain) in the chorio-allantoic fluid of chick embryos incubated for various periods at 35, 37 and 39 C. The rate of virus increase in the first 24 hours of incubation at 35 C was essentially the same as that in the same period at 37 C. The maximal level of infectivity at 35 C, reached at the 48-hour period of incubation, was about six times that at 37 C attained at 36 to 48 hours' incubation. Hemagglutinative activity seemed definitely greater at 35 C than at 37 C. From the present experiments it is evident that the temperature 39 C was unsuitable for the cultivation of influenzal virus B. Not only was the ultimate level of the total amount of virus low, but the infectivity and hemagglutinative activity found in the earlier periods of incubation at this temperature were rapidly lost later.

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This work was aided by the Dorothy Beard Research Fund and by a grant to Duke University from Lederle Laboratories, Inc., Pearl River, N. Y. The investigation was also supported through the Commission on Acute Respiratory Diseases, Board for the Investigation and Control of Influenza and Other Epidemic Diseases in the Army, Preventive Medicine Division, Office of The Surgeon General, United States Army, and by grants from the Commonwealth Fund, the W. K. Kellogg Foundation, the John and Mary R. Markle Foundation, and the International Health Division of the Rockefeller Foundation to the Board for the Investigation and Control of Influenza and Other Epidemic Diseases for the Commission on Acute Respiratory Diseases.

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