The 1918 influenza pandemic caused over 40 million deaths and likely resulted from introduction and adaptation of a novel avian-like virus. Influenza A virus hemagglutinins are important in host-switching and virulence. Avian adapted influenza virus hemagglutinins bind sialic acid receptors linked via 2-3 glycosidic bonds, while human adapted hemagglutinins bind 2-6 receptors. Sequence analysis of 1918 isolates showed hemagglutinin genes with 2-6 or mixed 2-6/2-3 binding. To characterize the role of sialic acid binding specificity of the 1918 hemagglutinin, we evaluated in mice chimeric influenza viruses expressing wild-type and mutant hemagglutinin genes from avian and 1918 strains with differing receptor specificities. Viruses expressing 1918 hemagglutinin possessing either 2-6, 2-3 or 2-3/2-6 sialic acid specificity were fatal in mice with similar pathology and cellular tropism. Changing 2-3 to 2-6 binding specificity did not increase lethality of an avian adapted hemagglutinin. Thus, the 1918 hemagglutinin contains murine virulence determinants independent of receptor binding specificity.