A synthetic density gradient technique has been applied to the study of the PR8 and Lee strains of influenza virus in the angle centrifuge. The method counteracted convective disturbances and permitted about a fiftyfold improvement in clearing supernatant fluids of virus. Sedimenting boundaries of infective virus particles, hemagglutinin, and complement-fixing antigen were obtained in the angle centrifuge and correlated with boundaries observed optically in the ultracentrifuge. The sedimentation constant of infective Lee virus particles is approximately 800 Svedberg units, while that of PR8 virus is only about 700. On the assumption of spherical shape, these values correspond to approximate diameters of 85 and 80 mµ respectively. These values agree with those obtained by filtration with graded collodion membranes. The concentration of primary virus particles in untreated allantoic fluid preparations of PR8 or Lee virus is of the order of 0.01 per cent.

The primary infective particles are identical with the hemagglutinin and the complement-fixing antigen to a large extent. However, allantoic fluid preparations of PR8 virus also show a slightly inhomogeneous group of particles with an average sedimentation constant of 460 S, which are adsorbed by and eluted from red blood cells yet appear to be non-infective. In addition the virus preparations contain a small amount of "soluble antigen" which sediments more slowly than the virus and is not adsorbed by red blood cells. This soluble antigen is probably associated with material which was observed optically in the ultracentrifuge to sediment at rates ranging from very low values up to that characteristic of the primary virus boundary. This distribution of rate makes it seem likely that the material represents disintegrated virus particles.

Calculations based on the experimental results obtained indicate that of the order of 10 influenza virus particles are required to produce infection of chick embryos or mice with the PR8 virus. While a comparable number is required with Lee virus for infection of chick embryos, about 10,000 particles are necessary for infection of mice. The ratio of hemagglutinin to red blood cells required to produce 50 per cent agglutination with dilute virus suspensions in the standard test is roughly 1.

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