Two kinds of virus particles have been found in varying proportions in influenza A2 strains isolated during the 1957 pandemic. Pure populations of the different particles were obtained, and these substrains were genetically stable on serial passage in the chick embryo.
The two virus particles differ markedly in several biological properties though they are antigenically similar. One kind of particle, designated "+," is relatively sensitive to specific antibody, is highly sensitive to inhibition by serum inhibitors and urinary mucoprotein, fails to elute or elutes very slowly from human erythrocytes, and is capable of agglutinating erythrocytes treated extensively with V. cholerae filtrate. The other particle, designated "-," is relatively insensitive to antibodies and urinary mucoprotein, completely insensitive to serum inhibitors, elutes rapidly from erythrocytes, and can agglutinate erythrocytes treated extensively with V. cholerae filtrate. Both "+" and "-" particles destroy virus receptors on urinary mucoprotein.
The relative proportions of these two particles determine the characteristics of parent strains in reactions with specific antibody, mucoprotein inhibitors, and erythrocytes. The "+" and "-" particles with several easily identifiable markers are well suited for genetic studies.