Sera from patients with nephropathia epidemica (NE) or Korean hemorrhagic fever (KHF) were tested for specific antibody response to antigens of Hallnas virus and Hantaan virus strain 76-1 18. A Vero E6 derived cell line persistently infected with Hallnas virus strain B1, and Vero E6 cells freshly infected with Hantaan virus type strain 76-1 18 were used as antigens in the immunofluorescence assay (IFA) and the immunoblot. Blots were prepared from whole cell lysates. The convalescent-phase sera of NE patients tested in this study regularly revealed a marked reaction with a 52 kilodalton (Kd) protein of Hallnas virus and a 50 Kd protein of Hantaan virus. A convalescent serum from a patient with Korean hemorrhagic fever and a rat antiserum against Hantaan virus could recognize the 50 Kd band of Hantaan virus but showed n o apparent reactivity with the 52 Kd component of Hallnas virus in the standard dilutions. Some sera could additionally identify minor bands in the 55 Kd and/or 67 Kd region of the blots. A one-way cross reactivity between Hantaan and Hallnas viruses was also evident from the results of the immunofluorescence assays in that NE convalescent sera reacted with both viruses, whereas KHF convalescent or anti-Hantaan sera gave strongly positive results with Hantaan virus but only faint reaction with Hallnas virus.