The antigen of HFRS virus was demonstrated by means of the indirect fluorescent antibody procedure in the lung tissue of bank and redbacked voles (Clethrionomys glareolus, Cl. rutilus) trapped in HFRS foci in the European part of USSR. This antigen has been used satisfactorily for serodiagnosis of HFRS in several European and Asian regions of the USSR where HFRS had been found to be endemic.