HFRS was confirmed serologically in two patients, who were infected in the natural focus near Zupanja. Using the bigger number of virus antigens in indirect immunofluorescent antibody test, afterwards was proved the agent could be classified into the serotype 3 of Hantaviruses, and antigenically is identical with virus NE (strain Hällnäs), respectively Yugoslav isolate "Vranica". Serotype differentiation of the agent in patient who were infected in the area of Igman, near Sarajevo, has not been performed by the same test. There was assumed a possibility for circulation of "new", up to now non-identified, serovariant of Hantaviruses in this natural focus.