The possibility of using indirect hemagglutination test (IHA) with erythrocyte immunoglobulin diagnosticum for detection of CHF virus in the blood of patients early after the onset was studied. It was shown that viremia could be detected by the IHA test within 2-3 hours provided the blood was collected not later that 8 days after the onset of the disease. The sensitivity of the test for the detection of viremia is as high as that of bioassays. The IHA test may also be used for the detection of CHF virus specific antigen in cadaver materials (blood, organ suspensions) even in those cases when virus isolation is prevented by toxicity of the specimens.