Pregnant inbred JY-9 strain guinea pigs (GPs) were infected with GPCMV in early gestation. Pooled hyperimmune serum from non-infected animals previously immunized with glycoprotein b (gB) was used for passive immunization 24h post-infection in half the pregnant dams. Placentae and pups from dams administered nonimmune (NI) sera were smaller in size and weight. All maternal blood cultures were negative on day 4 post-infection. All blood cultures were positive on day 7 and 14 in NI dams. All blood cultures were positive on day 7 and ∼50% positive on day 14 in passively immunized (PI) dams. Placental tissue cultures were positive in ∼50% of all animals. NI dams had ∼25% risk of pregnancy loss by in utero resorption, abortion and maternal death. No fetuses from the NI dams were of normal weight or size. Approximately 50% of PI dams had premature deliveries, but no abortions or maternal death. On HSE stains, all placentae showed features of immaturity consistent with midterm gestation. There was marked tabular congestion with focal mononuclear cell infiltrates in the lobules of placentae from NI dams. Foci of necrosis with neutrophilic infiltrates were seen in the trophoblastic septae of NI placentae. There was less severe lobular and septal mononuclear infiltration in placentae from PI dams. No necrosis or neutrophilic infiltrates were seen in PI placentae. Immunoperoxidase stains on frozen sections of placentae showed less disorganization of architecture and decreased numbers of infected cells in PI dams. Passive immunization initiated 24h post infection using anti-gB serum appears to have some protective effects on early pregnancy and the immature placenta in this model.