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The biophysical properties of several preparations of PR8 influenza virus have been studied. Electron micrographs showed slightly irregular, circular particles with an average diameter of 115 mµ and a standard deviation of the distribution of diameters of 15 per cent. The specific volume was determined with a pycnometer to be 0.79. The sedimentation rate was found to vary inversely with the concentration of virus. The extrapolated value for one preparation was 722 Svedberg units and the value for another preparation was 658 units. Sedimentation studies in sucrose solutions of varying densities showed a non-linear dependence of sedimentation rate upon solvent density, indicating that the density in solution increases with increasing sugar concentration. The virus particles floated in a sugar solution with a density of 1.18. The density in the absence of sucrose was estimated to be about 1.1. The virus was shown not to lose infectivity very rapidly in sucrose solutions. The spreading of the boundary during sedimentation was shown to be represented by a standard deviation of the sedimentation rate equal to 8 per cent of the mean. This corresponds to a distribution of diameters with a standard deviation of 4 per cent of the mean. A more slowly sedimenting component was observed with the ultracentrifuge in the highest concentrations of several of the preparations. The intrinsic viscosities, calculated on the basis of concentrations expressed as grams per cubic centimeter, of several highly purified preparations were determined to be between 11.3 and 16.5. The variation of sedimentation rate with concentration was shown to be due entirely to the variation of solution viscosity with concentration. The viscosity of the virus preparations can be explained as being due in great part to the presence of the slower sedimenting contaminating components which possess a very high intrinsic viscosity. The average particle diameter of the anhydrous PR8 virus particle was calculated from sedimentation and specific volume data to be about 80 mµ. The discrepancy between this value and that obtained from the electron micrographs, the greater size distribution from the electron micrographs, the slight irregularities in some of the particles as observed in the electron micrographs, the behavior of the sedimentation process in sucrose solutions of different densities, and the inactivation of the virus by withdrawal of electrolytes can all be explained in a straightforward manner if it is assumed that the virus particles in solution contain, in addition to the constituents shown by chemical analysis, about 60 per cent by weight of water. It is estimated that such hydrated virus particles are about 100 mµ in diameter.
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