An indirect hemagglutination (IHA) test with Congo and Crimean hemorrhagic fever (CHF) viruses employing sheep erythrocytes sensitized with immune γ-globulin was developed. The titer of the IHA test with sucrose acetone antigens of the virus reached 1:16,000–1:32,000; the reaction was specifically inhibited in the presence of immune sera and sera of CHF-convalescent patients. High sensitivity and specificity of the reaction recommend it for the serodiagnostics of CHF.