Chronic villitis is almost always present in intrauterine infection with human cytomegalovirus (HCMV). The inflammatory response to this virus has been described in detail. However, little is known about the types of placental cells that may be infected by HCMV and six cases of HCMV placentitis were thus investigated to identify the vulnerable cell types. Immunohistochemical double staining analyses were performed using antibodies to HCMV immediate early antigens and to specific cellular marker proteins. Fixed connective tissue cells could be demonstrated to be the predominantly infected cell type in each placental tissue. Endothelial cells and macrophages were also found to be infected in all six cases, whereas evidence of trophoblast infection was obtained in four cases. It is concluded that release of infectious virus by connective tissue cells, macrophages and endothelial cells may play a critical role in transplacental transmission of HCMV. The findings further suggest that the cytopathic effect of HCMV infection on these cells might be involved in the pathogenesis of intrauterine HCMV disease.