Lymph nodes, thymuses and bone marrows from SD rats inoculated with Hantaan virus were collected on 1, 2, 4, 6, 8, 10, 12, 20, 30 and 40 days after inoculation of Hantaan virus, and examined by indirect immunoluorescent antibody technique using monoclonal and polyclonal antibodies against Hantaan virus antigens to demonstrate the replication of Hantaan virus in centraI and peripheral immune organs. The results were summarized as follows. 1. Hantaan virus antigens were found in all the thymuses and bone marrows from the day 1 to 40 after inoculation of virus. 2. In the lymph nodes, antigens were found in different stage of infection according to the kinds of lymph node. Antigens were demonstrated on the day 30 and 40 in submandibular nodes, from the day 14 to 40 in parabronchial nodes, from the day 6 to 40 in rnesenteric nodes, and only on the day 14 in iliac nodes. 3. There were no differences of the results between using monoclonal antibody and polyclonal antibody against Hantaan virus antigens. 4. Hantaan virus antigens were not found in tissues from normal SD rats. 5. Identification of cells which had Hantaan virus antigens were not possible under the fluorescent microscope.