Serological relationships between neuraminidases of representative strains of influenza virus isolated between 1930 and 1967 were studied, using rabbit antisera. The neuraminidases of type A strains formed three antigenic groups, , A0–A1 and A2, with antigenic drift within these groups. The neuraminidases of type B strains were unrelated to those of type A strains, and also showed antigenic drift. Four subgroups could be distinguished in haemagglutination-inhibition tests, , A0, A1 and A2. Antigenic drift occurred within each of these subgroups; antigenic changes in the neuraminidase and haemagglutinin occurred independently.

The neuraminidases of different strains as well as variants of a single strain varied greatly in their heat stability, which had no direct relation to the antigenic changes or the time of isolation of the strains. Both the antigenicity and the enzymic activity of the neuraminidases of certain strains () were heat-labile and it was necessary to use freshly prepared virus suspensions for preparing antisera to their neuraminidases.


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