To determine whether placenta and plasma of preeclamptic women contain factors that cause endothelial cell damage. Placental extracts and plasma from preeclamptic and normotensive women were added to cultures of normal human umbilical vein endothelial cells and their effect on their viability, was determined by MTT reduction and 51chromium release. Placental extracts from normotensive and preeclamptic women were cytotoxic to endothelial cells, but not the plasma from both groups. Mean +/- standard deviation values of cytotoxicity index in preeclamptic and normotensive placental extracts using the MTT reduction were 70.3 +/- 6.76% and 51.4 +/- 8.81%, respectively, showing a significant difference (P < .0001). Using the 51chromium-release assay, preeclamptic placental extracts showed cytotoxic effects of 87.6 +/- 13.47% compared with 17 +/- 20.60% in control patients. The cytotoxic activity decreased after trypsin digestion and heat treatment in both groups. A cytotoxic factor to endothelial cells in placental extracts of preeclamptic women was identified. This compound is thermolabile and sensitive to trypsin digestion.