Poly(ethylenimine) (PEI) was used to transfect the endothelial cell line EA.hy 926, and the secreted levels of three gene products, tissue-type plasminogen activator (tPA), plasminogen activator inhibitor type 1 (PAI-1), and von Willebrand Factor (vWF), were assessed via ELISA. We found that the levels of these gene products in cell supernatants increased by factors up to 16.3 (tPA), 8.3 (PAI-1), or 6.7 (vWF) times the levels recorded for untreated cells, and roughly correlated with the percentage of cells that expressed the reporter plasmid. Transfections carried out using promotorless constructs of the same reporter plasmid also yielded increases in tPA, PAI-1, and vWF to similar extents. Additionally, data regarding cell viability were gathered and found to inversely relate to both the effectiveness of the PEI used for transfection and the secreted levels of the three mentioned products. There appeared to be two distinct types of cell death, resulting from the use of either free PEI (which acts within 2 h) or PEI/DNA complexes (which cause death 7-9 h after transfection). Cells were also transfected by poly(L-lysine) and liposomal carriers, and increases in secreted tPA similar to those seen with PEI-mediated transfection were observed for positively transfected cells. The results of these investigations indicate that non-viral gene delivery can induce a state of endothelial cell dysfunction, and that PEI-mediated transfection can lead to two distinct types of cell death.