Influenza virus can survive, and can be propagated in immunological environments induced in mice by vaccination with the homologous strain of virus: survival was associated with the emergence of variants which differed from the parent strain in antigenic characteristics.

The data concerning hemagglutinating activity of the variants, on the one hand, and of the antigenicity, on the other, are compatible with the concept that the structure of the influenza virus includes a surface arrangement which is distinct from the inner virus bulk.

The points (a) that propagation was accomplished with difficulty whenever the immunological environment was altered, and (b) that once established, passage was continued without difficulty, are interpreted to indicate that the mechanism of variation may involve a rearrangement of the basic hereditary mechanism.

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