Hyperimmune rabbit sera were examined by specific adsorption tests for influenza viruses H1N1 isolated in 1942-1952. A comparative immunological analysis revealed the presence of three antigenic determinants in hemagglutinins (HA) of A/Bel/42, A/Weiss/43, A/FM/1/47 and A/FLW/52 viruses. Adsorption of antisera with different drift-variants of H1N1 produced monoreceptor sera to individual antigenic determinants of HA by means of which marker antigens can be identified in HA of viruses isolated in different periods, and one may have an idea of evolutionary variability of influenza virus.