Terminal complement complex and S protein were searched for in term placentae obtained from 13 women with normal pregnancy and 15 patients with moderate or severe form of pre-eclampsia. Terminal complement complex was found to localize in the fibrinoid material of the decidua of the basal plate, in the stroma of the chorionic villi and in the vessel walls, as subendothelial deposits. S protein had a quite different distribution, being detected in the syncytiotrophoblast located both in the chorionic villi and in the decidua of the basal plate (DBP) and also on the endothelial cells of fetal stem vessels. Mild deposits of C3 were found in the decidua of the basal plate and also in the stroma and on the basal membranes of the villi. Reactivity for C9 neoantigen was also observed in the cytoplasm of some cells, which were recognized to be macrophages by the presence in their cytoplasm of acid phosphatase and by their reaction with a monoclonal antibody specific for macrophages. Differences in complement deposition in normal and pre-eclamptic placentae were essentially quantitative. Possible mechanisms of complement activation are discussed.