The pathogenic agent of influenza A has been sedimented from infected extra-embryonic fluids of the developing chick, embryo by ultracentrifugation. Material so obtained contains two fractions resolvable in the analytical centrifuge cell. The first, a homogeneous fraction, showed a sedimentation constant S20 = 20 to 31 x 10–13. The second showed a sedimentation constant S20 = about 800 ± 100 x 10–13, was much less homogeneous than the first, and was shown to consist principally of aggregated particles of the more disperse fraction. Both fractions contained the virus in essentially equal amounts per unit of protein weight as calculated from nitrogen determinations.
Electron micrographs of the isolated virus protein indicated that the predominating unit is roughly spherical in shape and has a modal particle diameter of about 11 mµ, in good agreement with the sedimentation data in indicating a molecular weight of about 650,000.
Approximately 300 of the particles having molecular weight of 650,000 were present in the minimal dose producing infection in mice after nasal instillation.
The influenza A virus may now be regarded as one of the smallest pathogenic agents thus far isolated. Preliminary analyses indicate that it is also one of the least complex, being composed principally of nucleoprotein.