Consecutive serum samples collected from 235 patients with Haemorrhagic Fever with Renal Syndrome (HFRS), between two days and two years after onset of disease, have been analysed for the presence of IgG and IgM type of antibodies specific for Hanta-viruses. The sera were screened in parallel by a newly developed indirect Immuno Enzyme Assay (EIA) in parallel with Indirect Immunofluorescent Antibody Assay (IFA). In both tests the Hantaan virus strain 76-118 was used as the antigen. The EIA was much more sensitive than the IFA test for the detection of IgM type antibodies. With the indirect EIA IgM type antibodies against Hantaan virus 76-118 have been detected in HFRS patient's sera from the second day of illness indicating the usefulness of this test for the early serological diagnosis of this disease.