The effects of maternal-infant transfer of specific antibody on infection attributable to hemorrhagic fever with renal syndrome-causing virus in the infant rat were studied. In cross-fostering experiments, maternal hantavirus-specific antibody was shown to be transferred equally effectively to infants either in utero or by breastfeeding. Both IgG- and IgA-specific antibodies were transferred. Infected infants that initially acquired high levels of maternal antibodies by either route survived lethal doses of the virus and did not show any signs of disease. Furthermore, 15 weeks later, these animals had no evidence of remaining antibodies or viral infection. These results indicate that maternal antibody, transferred to the infant rat in utero or by breastfeeding, is protective against hantavirus infection.