In order to determine the conditions for the optimum production of PR8 influenza virus in chick embryos, a study has been made of the róles of concentration of virus in the inoculum, temperature of incubation of infected embryos, length of time of incubation of infected embryos, and age of embryos at the time of inoculation. Relative amounts of virus in different preparations were measured indirectly by means of determinations of chicken red blood cell agglutination titers. Frozen infectious allantoic fluid which produced infection in chick embryos at a maximum dilution of 10–7 was employed as a stock inoculum. Best results were obtained with an amount of stock inoculum of 0.1 cc. of a 10–5 dilution, a temperature of incubation of 35°C., a length of time of incubation of 36 to 48 hours, and with embryos brought to 10 or 11 days of age at 37°C. or 9 or 10 days of age at 39°. An uncontrolled factor arising from inherent variations in the properties of different embryos and different batches of embryos was discussed.

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