Eight strains of influenza virus were titrated as plaque-forming units in both monolayers and suspensions of chick embryo cells. In the absence of trypsin, satisfactory plaques were formed only by the A/WSN strain of virus. When trypsin was included in the overlay medium of cell monolayers, all the influenza virus strains tested produced plaques, and the plaque infectivities of all but one strain were close to the egg infectivities. Using cells suspended in agar in the presence of trypsin, four strains gave plaque infectivities indistinguishable from egg infectivities, two strains formed plaques with rather low efficiency and two strains did not produce plaques. Plaque formation was also enhanced by pronase or subtilisin. A preliminary investigation was made of the mode of action of trypsin.


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