Rabbit antisera were prepared against ten antigenically different influenza A strains. These sera were absorbed with one or more heterologous strains and in each case all the heterologous or crossing antibody was removed; the anti-bodies remaining after this treatment were specific for the immunizing strain or group of strains.
On the basis of reactions with absorbed sera, the strains fell into seven groups.
Absorbed specific antisera of these groups were used to test the HI titer against a large number of influenza A viruses. Most of the strains were inhibited by a single serum, a few were inhibited by none of the sera, and only one strain was inhibited by two antisera.
The grouping of strains by this method was less equivocal than classifications based on previous tests. When more fully developed, this technique promises to be of interest and assistance in the study of influenza, especially from the epidemiological and prophylactic standpoints.