A direct ELISA technique was used for demonstration of HFRS virus antigen in suspensions of the lungs obtained from 774 wild small mammals belonging to 13 zoological species trapped in 21 localities in Czechoslovakia between 1983 and 1984. Western serotype of HFRS virus was demonstrated in the lungs of 8 Microtus arvalis voles caught in the areas of central Bohemia, southern Moravia and western Slovakia, and in one Clethrionomys glareolus vole from the area of eastern Slovakia. The eastern serotype antigen was found in one Apodemus agrarius field mouse captured in the area of eastern Slovakia. The antigen titres as determined in the lungs of these rodents by the ELISA and RIA techniques were relatively high. Human sera examined for the presence of HFRS-specific antibody by the indirect fluorescence and RIA techniques were 76 serum specimens from chronic kidney patients treated by hemodialysis and 349 sera of adult males over 35 years of age coming from various districts of Bohemia and Moravia. Antibodies against HFRS virus antigen were found in 4 sera obtained from northern Bohemian men. The results provide the evidence which suggests that the natural foci of HFRS are likely to occur across much of the Czechoslovak territory, and confirms the focality of this infection in at least some of the localities characteristic in the 1950s and 1960s of the incidence of severe sporadic cases of HFRS.