AbstractTransplacental infections with MVM were studied in hamsters, rats, and mice. The most striking infections were in hamsters in which infected fetuses developed high titers of virus, then died, and were resorbed. Only a few fetuses out of each litter became infected, the majority remaining alive, well, and free of infection. The situation in pregnant mice, which are natural hosts of this agent, was different. In them parenteral inoculations produced no discernible effects on either mother or fetuses. But infected fetuses with high titers of virus were recovered from other females at term. In neither hamsters nor mice were instances found of placental infection unaccompanied by fetal infection. Why only a small percent of inoculated pregnant mice developed transplacental infections was not clear. Rats free of immunity and hence susceptible to parenteral inoculations were not found. Intrafetal inoculations of rats revealed, however, that MVM can proliferate in fetuses and then persist for weeks following birh without inducing detectable illness. Histologic studies revealed sparse but characteristic intranuclear inclusions in some animals but it was evident in others that MVM can be present in substantial titers in an absence of apparent histologic change. After direct inoculation of some rat and hamster fetuses generalized severe disease resulted, with replicating tissues being the major viral targets, as has been previously observed in infections with other intrauterine parvoviruses. Here large infectious doses were thought to be responsible for the special effects observed.