The mechanism of transplacental infection has been investigated by using IF microscopy to identify infected cells in maternal and fetal tissues at progressively longer intervals after maternal oronasal exposure. Examination of tissues contiguous with the maternal-fetal junction revealed viral antigen in endothelial and mesenchymal cells of the chorion, with increasing involvement of these tissues at progressively later stages of gestation. Viral antigen was never detected unequivocally in either uterine epithelium or trophectoderm. Consequently, there was no evidence for maternalfetal transfer of the virus by replicating through these tissues. However, this route cannot be excluded, since only a small part of the total area of contact was examined. Transfer of the virus within macrophages has been considered. Whatever the route, maternal viremia seems a likely prerequisite for transplacental infection.