Embryonic chick, one-day-old chick, adult chicken, duck, pigeon, turkey, guinea-pig and human erythrocytes may be agglutinated in the Hirst reaction while rabbit, rat, mouse and sheep are not.
Chicken and duck cells have given the highest titers with infected allantoic fluid whereas pigeon and human have given the lowest.
Infected mouse-lung may be used as the source of virus in either agglutinative or agglutination-inhibitive tests. The results parallel the use of allantoic fluid.
Filtration removes much of the infectious agent by both mechanical blocking and adsorption. A rough estimate may be made of the degree of this adsorption of influenzal virus by the in vitro test.
The extent of consolidation of the mouse-lung, pathogenicity for mice and agglutinative titer seem to be closely associated. Bacterial contamination of lungs does not destroy the agglutinating principle.
The PR8 strain has been found by these means to be serologically related to the Alaska strain but does not appear related to the McCallum or PJP strains.