In congenital cytomegalovirus (CMV) infection histologic evaluation of the placenta is often unrevealing. In the present study immunocytochemistry to CMV immediate early and early nuclear antigens was used to characterize placental involvement in six cases of symptomatic intrauterine CMV infection. Histologic examination had demonstrated diagnostic viral inclusions in one placenta and non-specific villitis in another. However, immunocytochemistry revealed CMV infection in five of the six placentas, including three with no pathologic changes on routine histologic evaluation. Infected cells were located primarily in the villous stroma. In one case immunoperoxidase staining showed infection in the syncytiotrophoblast. Infected endothelial cells were demonstrated by double staining for CMV and factor VIII antigen. No double-stained cells were seen in tissue sections stained for CMV immediate early nuclear antigen or the human macrophage-associated CD68 antigen, which is expressed in Hofbauer cells. In conclusion, specific immunoperoxidase staining was more sensitive for demonstrating placental CMV infection than was histologic examination and it aided in the characterization of infected cells.