For the first time in the study of epizootology of hemorrhagic fever with the renal syndrome (HFRS) in wild animals data are presented indicating certain relationship between true infection rate of bank voles with HFRS virus and the content of the circulating antigen of HFRS virus and antibody to this virus in these animals. Different age groups of animal populations were shown to have different intensity of immunity: it was more marked in animals of older age groups, and less in young animals. On the contrary, live virus was most frequently found in young animals. It has been shown that for successful isolation of the virus from bank voles it is necessary to take into consideration the characteristics of acute infection in rodents: the presence of HFRS virus antigen in their lung tissue and the lack of antibody to the virus in the blood.