Different virus isolates causing hemorrhagic fever with renal syndrome (HFRS) were compared using a neutralization test. Patient convalescent sera and antisera prepared in rabbits were used to compare Puumala-related Hantavirus isolates from Finland, Sweden, Belgium, and the USSR. The majority of European isolates were indistinguishable from each other using both homologous rabbit antisera and patient convalescent sera. The European isolates of HFRS were also compared with prototype Hantaan (the etiologic agent of Korean hemorrhagic fever). The one-way cross-reaction between the Hantaan and Puumala viruses, previously described using human convalescent sera tested by indirect immunofluorescence and immunoprecipitation, was also seen by the neutralization test.