Yellow-green fluorescence representing viral antigens was detected in both the nucleus and cytoplasm of epithelial cells of the respiratory tract in ferrets infected with influenza virus. This nuclear fluorescence was the chief manifestation of cross-fluorescent staining reactions among three strains of influenza A virus studied, PR8, Farrington, and Fm1. Absorption experiments with influenza viral V and soluble S antigens showed that S antigen was responsible for the presence of fluorescence in the nucleus and for the cross-staining reactions among these strains.

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