Abstract
The neuraminidases and haemagglutinins of 20 isolates of swine influenza virus collected between 1930 and 1967 were compared in neuraminidase-inhibition and haemagglutination-inhibition serological tests. All the isolates were serologically related to a greater or lesser extent in tests with either antigen. The antigenicity of the two antigens varied independently in different isolates. There was a slight, but statistically significant, drift in the antigenicity of both antigens during the period in which the isolates were collected.
* Present address: Institut für Hygiene und Med. Mikrobiologie der Technischen Hochschule München, Federal Republic of Germany.