Antibodies to hantavirus, Seoul type B-1 strain, vertically transferred to rat neonates prevented lethal as well as persistent infection. When relatively high titer viruses were inoculated into neonates, the mother's antibodies protected all the neonates from lethal virus infection. However, the antibodies could not protect all of the neonates from persistent infection but only half of them underwent persistent infection. The other half was completely cured but also became persistently infected when rechallenged with the active viruses after reaching maturity.