Titers of hemorrhagic fever with renal syndrome virus were estimated by counting foci stained with peroxidase-antiperoxidase or by using immunofluorescence methods. Foci stained with PAP were clearer and easier to count. The peroxidase-antiperoxidase method showed a linear relationship between virus concentration and the number of foci. A focus-reduction neutralizing antibody test was performed with immune rat sera and sera from patients. Again, the peroxidase-antiperoxidase method proved to be more convenient and reliable than the immunofluorescence method. Antigenic differences between Hantaan virus, 76-118 and hemorrhagic fever with renal syndrome virus B-1 were clearly demonstrated by the neutralizing antibody test.