Inbred JY-9 strain guinea pigs (gp) were infected with gpCMV in early pregnancy. 7 pregnant dams were treated with pooled hyperimmune serum from non-infected animals previously immunized with glycoprotein B (gB), on days 0, 2 and 5, day 1 being the day of infection. 6 control pregnant dams received pooled non-immune(NI) serum as above. NI dams appeared sicker and had 83% pregnancy loss by in utero resorption and abortion compared to 14% in passively immunized (PI) dams(p<0.01). Mean placental weight was 4.3g in NI dams and 5.6g in PI dams(p=0.02). Mean pup weight was 9.8g in NI dams and 31.5g in PI dams(p<0.02). Mean pup length was 3.7cm in NI dams vs. 6.6cm in PI dams(p<0.02). All maternal blood cultures were positive on day 7 post-infection. On day 12-14, 83% of NI dams vs. 28% PI dams had positive blood cultures(p<0.05). Placental tissue cultures were positive in 80% of NI dams compared to 33% of PI dams(p<0.05). Pup tissue cultures were positive in 50% of NI dams compared to 25% of PI dams(NS). All placentae showed features of immaturity consistent with mid-term gestation, on H&E stains. NI placentae had marked lobular congestion with focal mononuclear cell infiltrates in the lobules and foci of necrosis with neutrophilic infiltrates in the trophoblastic septae. Less severe mononuclear cell infiltrates and no necrosis or neutrophilic infiltrates were seen in PI placentae. Frozen sections of PI placentae showed less disorganization of structure with decreased number of infected cells by immunoperoxidase stains. Passive transfer of anti-gB serum initiated 24 hours pre-infection does not prevent, but significantly modifies the effects of primary maternal CMV.