The interrelations between H3/73 hemagglutinin of human influenza virus and the other 16 mammalian and avian hemagglutinin subtypes (a total of 50 strains) were studied by the method of radioimmunologic analysis (RIA). The antigenic relations of H3, Hav7 and Heq2 were confirmed, certain common determinants were also found in H3/73 hemagglutinin and avian viral Hav6 and Hav9 hemagglutinins. No interrelations were revealed with previously circulating human influenza viruses H0, H1, H2 as well as with swine influenza virus and avian viruses Hav1-Hav5, Hav8. It has been shown that the H3/73 determinant in some avian viruses evolves similarly to drift-variants of human influenza virus. The method can be recommended for fine analysis of influenza virus antigenic structure as it allows detecting small antigenic quantities.