We previously hypothesized that the endothelial cell dysfunction observed in women with preeclampsia might be caused by an imbalance between circulating very low density lipoproteins and a cytoprotective pI 5.6 isoform of albumin, referred to as toxicity preventing albumin (TxPA). An accurate simplified method was developed to quantify TxPA in small volumes of pregnancy plasma by gel electrofocusing. This assay revealed that circulating TxPA concentrations in women with severe preeclampsia were significantly reduced compared to those in normal pregnant women and women with benign transient hypertension of pregnancy. Nonesterified fatty acids (NEFA) and triglycerides were elevated in plasma from women with severe preeclampsia compared to those in plasma from the two control groups. The inverse correlation between TxPA and NEFA values led us to analyze the NEFA bound to plasma albumin. Gas chromatography and mass spectrometry demonstrated no qualitative differences in the specific fatty acids bound to plasma albumin in severe preeclamptic and normal pregnant women. However, the quantity of NEFA bound to albumin was greater in preeclampsia plasma (2.5 mol NEFA/mol albumin) compared to that in normal pregnancy plasma (0.8 mol NEFA/mol albumin), accounting for the acidic pI shift observed in albumin from the former patients. Functional assays demonstrated that human very low density lipoprotein particles were toxic to human umbilical vein endothelial cells in vitro, but this toxicity was prevented by the addition of TxPA albumin to the culture medium.