Light- and fluorescence-microscopy examinations of 186 placentas were carried out, with concurrent determination of placental interferon, isolation of influenza virus, and serologic tests of maternal and fetal blood, and amniotic fluid. In 32 of the cases, placentitis caused by serotype A or B of influenza virus was present, characterized by hyperplasia and subsequent destruction of amniotic cells, trophoblast, decidual cells, and vascular endothelium, by the presence of influenza antigens, fuchsinophilic cytoplasmic inclusions in the affected cells, and lymphoid infiltrates, and/or by circulatory disturbances. In addition, an interferon possessing properties of viral, immune, and placental interferon was detected in a number of placentas. A or B influenza virus was isolated from 3 placentas. Diagnostic titers of anti-influenza antibody occurred in fetal blood and amniotic fluid samples. The present results indicate that influenza virus may persist and replicate in placental cells.