Transmission of mouse hepatitis virus (MHV) in utero following oronasal inoculation of pregnant mice was found to depend upon MHV strain and host genotype. Virulent, polytropic MHV-JHM was recovered from multiple maternal tissues, including liver and uterus, as well as placenta and fetus in susceptible BALB/cByJ mice. Fetuses were infected during all 3 trimesters of pregnancy. Low virulence, polytropic MHV-S infected fetuses in a low percentage of susceptible BALB/cByJ dams. Infection of resistant CD-1 mice with MHV-JHM was limited, with no fetal infection. Enterotropic MHV-Y was largely restricted to intestine of BALB/cByJ and CD-1 dams, with minimal dissemination and no fetal infection. Maternally-derived MHV IgG antibody was detectable in pup sera through 4 weeks of age. Antibody titers were generally lower in second litters of the same dam. Cross-fostering experiments showed that antibody was transferred via colostrum and not in utero, and that pups were capable of absorption through 2 weeks of age. Pups nursing immune dams were protected against MHV challenge at 1 and 2 weeks of age, compared to pups nursing naive dams. Immunity to MHV challenge was cross-protective against both antigenically homotypic and heterotypic strains of MHV.