Hemagglutinins of Hantaan virus strains isolated in Southern Far East of Russia from patients with hemorrhagic fever with the renal syndrome and from rodents, natural vectors of the virus, were studied in the cross kinetic hemagglutination inhibition test with immune sera to the studied strains. The strains differed by the functional activity index. Use of hemagglutinins of functionally active Hantaan strains in serologic studies notably improves the diagnostic efficacy of the hemagglutination test in Hantavirus infection.