Freezing of the chorioallantoic membrane after brief fixation in formalin preserves antigenicity of cellular components and allows penetration of ferritin-conjugated antibody. Dense aggregates of viral antigen, presumed to be of the soluble type, were found in the nuclei of cells infected with influenza virus. Intracytoplasmic antigen, on the other hand, was widely dispersed. The experimental observations are consistent with the hypothesis that soluble antigen diffuses into the cytoplasm through intact nuclear membranes. Nucleoli do not appear to be the sites of antigen synthesis.

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