A rapidly frozen vitrified aqueous suspension of influenza A virus was observed by high resolution electron cryomicroscopy. The influenza particles were grouped into small (diameter < 150 nm) spherical particles with well organized interiors, large spherical ones with less internal organization, and framentous ones. Envelopes of most of the large virus particles were phospholipid bilayers, and the chromatography fraction containing these large particles was largely devoid of viral activity. The envelopes of most of the fiamentous and small spherical virus particles, on the other hand, gave a strange contrast which could be ascribed to a combination of a thin outer lipid monolayer and a 7.2 nm thick protein-containing inner layer. These latter particles represented most of the viral activity in the preparation. Densitometric traces of the near in-focus images confimed these structural differences. Some viral envelope structures apparently intermediate between these two distinct types of membrane were also detected. A structural model of intact biologically active influenza virus particles was formulated from these results, together with computer simulations.

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