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, sw, 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, sw, A0, A1 and A2. Antigenic drift occurred within each of these subgroups; antigenic changes in the neuraminidase and haemagglutinin occurred independently.