Each of 5 gilts was vaccinated IM with modified live-virus (MLV) vaccine for porcine parvovirus (PPV), and 5 gilts were used as nonvaccinated controls. Vaccinated gilts developed hemagglutination-inhibiting (HI) antibodies to PPV (titer of 320 to 1,280) by 2 weeks after vaccination. All gilts wee bred, and at about 40 days of gestation their immunity was challenged by intranasal and oral administration of a virulent strain of PPV. Gilts were killed at about 84 days of gestation and their litters were examined. Litters from vaccinated gilts comprised 59 live and 2 dead fetuses. Neither the virus nor antibodies to the virus were detected in any of the fetuses. In contrast, litters from nonvaccinated gilts comprised 25 live and 29 dead fetuses, and PPV was isolated from all dead and 9 live fetuses. Viral antigen was detected by direct immunofluorescence in the lungs of all PPV-infected fetuses, and 7 of the live infected fetuses had HI antibodies. Modified live virus did not cause transplacental fetal infection in 3 seronegative gilts inoculated at about 40 days of gestation. All 3 gilts had antibodies to PPV (80 to 640) when killed at about 84 days of gestation, and neither HI antibodies nor PPV were detected in any of the 20 live and 1 dead fetuses from these gilts. Modified live virus replicated in the tissues of fetuses of 2 gilts inoculated in utero. Seven of 8 fetuses inoculated with MLV died and were infected. These results demonstrated the efficacy of MLV vaccine for the prevention of PPV-induced reproductive disease in swine. Although MLV did not cross the placental barrier in pregnant animals, its pathogenicity for porcine fetuses after direct in utero inoculation indicates that its use should be limited to nonpregnant animals.