Studies on serodifferentiation by indirect immunofluorescence antibody method and neutralization test of Hantaan virus strains isolated from human patients and rodents revealed circulation on the Maritime Territory of 5 virus serotypes and demonstrated that rodents of one species could be virus carriers of different virus serotypes. On the basis of the antigenic similarity of strains isolated from human patients, field mice, and Clethrionomys rufocanus it is concluded that the virus serotypes circulating among rodents of these species may play a role in the etiology of cases of hemorrhagic fever with renal syndrome.