ABSTRACT: Monolayer cultures of human endothelial cells were incubated with pre- and postdelivery sera from five women with preeclampsia and four matched, normal pregnancies. Conditioned media collected from endothelial cells pretreated in vitro with prepartum sera from preeclamptic women contained greater mitogenic activity and elevated levels of platelet-derived growth factor (PDGF)-like peptides than cells exposed to normal pregnancy sera or postpartum preeclamptic sera. Under the same experimental conditions, predelivery preeclamptic sera stimulated greater expression of endothelial cell PDGF-B-chain mRNA than that accumulated in the presence of matched postdelivery sera. By contrast, no differences in endothelial cell PDGF-B mRNA levels were noted when pre- and postdelivery sera from normal parturients were tested. The results suggest that a factor(s) in the blood of preeclamptic women can stimulate the synthesis and release of a potent growth factor and vasoconstrictor from human endothelial cells.