SUMMARY: An influenza A strain, Ryan, isolated in Melbourne in 1954 contains on a particle basis about 20% filaments. By using the electron microscope for particle counting, the efficiency as haemagglutinin of Ryan 'long' filaments (particles in which length : width > 6) was estimated to be about 6 times greater than that of spheres of PR8 virus. Allantoic fluid preparations of Ryan virus have high values for the ratio (log) EID50:AD and estimates indicate that a maximum of 6 'long' filaments are equivalent to one EID50, compared with 18 particles in the case of P R 8 virus or 14 particles in the case of a mutant strain of Ryan virus which exists almost exclusively as spheres. Compared with virus PR 8 spheres (16 particles : AD), spherical particles present in filamentous Ryan preparations have a low efficiency as haemagglutinin (29 to 38 partic1es:AD). These particles are formed before the 'long' filaments, and in fluids harvested 21 hr. after inoculation when the proportion of 'long' filaments is very low, about 9 spherical particles constitute 1 EID50. For the purpose of subsequent chemical examinations filaments were isolated and concentrated by a process involving specific adsorption to and elution from red cells, followed by 3 cycles of differential centrifugation. The ratio of filaments: spheres in such preparations is about 50:50. 'Purified' filaments weigh about 30 times as much as the spherical particles present and account for over 95 % of the total weight present in purified preparations.

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