Immune adherence hemagglutination (IAHA) test was used to investigate the production of antibodies against Hantaan (HTN/83-138) and Seoul (SEO/80-39) viruses in sera from Hemorrhagic fever with renal syndrome (HFRS) patients who were infected with Hantaan virus in Korea. The five patients diagnosed as typical HFRS have been bled at weekly intervals starting from 3 days after onset of disease up to 66 days. Sera were tested to determine the titers of antibodies as well as the reactivities against Hantaan and Seoul viral antigens. The results are as follows; 1. IAHA antibodies against Hantaan viral antigen: The antibodies increased 3 days after the onset of disease and reached at peak around one week later. The antibody titers were risen rapidly and the maximum titers were reached at around 2 weeks after onset of the disease and persisted for 2-3 weeks. Thereafter the titers were decreased slowly. 2. IAHA antibodies against Seoul viral antigen: The antibodies were detectable 3 days after the onset of disease and the titers of antibody were reached peak at around one week later and persisted with same level for 2 months. However the titers of antibody were not as high as Hantaan viral antigen. 3. Reactivities of HFRS sera against Hantaan and Seoul viral antigens: The reactivity of Hantaan viral antigen was always 4-16 times higher than Seoul viral antigen in all sera tested except the sera from very early stage of illness. The clear differences in reactivity between Hantaan and Seoul viral antigens were observed in all sera used regardless of time of bleeding.