The hemagglutinin (HA) and neuraminidase (NA) external glycoprotein antigens of 
H1N1 and H3N2 subtypes of epidemiologically important influenza A viruses 
prevalent during recent decades were subjected to intensive antigenic analysis 
by four different methods. Prior to serological analysis with polyclonal rabbit 
antisera, HA and NA antigens of four viruses of each subtype were segregated by 
genetic reassortment to forestall nonspecific steric hindrance during 
antigen-antibody combination. This analysis has demonstrated that with respect 
to antigenic phenotype, HA and NA proteins have evolved at different rates. With 
H1N1 viruses, an arrest of significant evolution of the NA discordant with the 
continuing antigenic drift of HA was found in the 1980-1983 period. It is 
probable that the different and independent rates of evolution of HA and NA 
reflect the greater selective pressure of HA antibodies, which forces the more 
rapid emergence of HA escape mutants. The slower antigenic change found for NA 
further supports the potential for NA-specific infection-permissive immunization 
as a useful stratagem against influenza.