The neuraminidase, the membrane protein, the ribonucleoprotein (nucleocapsid protein) and the light and heavy polypeptide chains of the haemagglutinin subunits were isolated from particles of influenza virus type A0 (strain bel) and type B (strain lee). Each of these substances migrated as a single component during electrophoresis on SDS-polyacrylamide gels. The neuraminidase of type A2 influenza was isolated from the A0-A2 recombinant virus, X-7F1. This enzyme contained two polypeptides differing slightly in electrophoretic mobility.