The etiologic agents of haemorragic fever with ranal syndrom (HFRS) in Korea are Hantaan and Seoul virus in the genus Hantavirus, family Bunyaviridae. In order to elucidate the role of maternal immunity to Hantavirus infection in rats, the protective effect of the maternal antibody were studies by using rats experimentally infected with Seoul virus strain HR80-39. Antibody titers of sera and viral antigen against Seoul virus were investigated by indirect immunofluorscence antibody technique (IFA). The dam sera had IFA antibody titers ranging from 1:128 to 1:1,024 after parturition. In fetuses, IFA antibody titers ranged from 1: 16 to 1:64 just after birth, increased to peak titers ranged from 1:256 to 1:1,024 in the 2nd week after birth. Challenged newborn rats had IFA antibody titers ranging from 1:64 to 1:1,024 after inoculation. No viral antigen was detected in lungs or other organs of the newborn rats. The maternal antibody to Seoul virus was transferred prenatally through placenta and postnatally via colostrum from immune dams to their offspring. These results demonstrated that maternal antibody to Seoul virus was quite effective in protecting newborn rats against same virus infection.