Prion hypothesis postulates that the conformantionally changed isoform of prion, PrPsc, is the TSE infectious agent which propagates by serving as a template to convert host prion protein (PrPc) to additional molecules of PrPsc. It has been shown that 2/3 of PrPc in blood is present in plasma. Interestingly, also portion of TSE infectivity present in blood has been shown to reside in plasma. The source of plasma PrPc and TSE infectivity is not known. We used monoclonal antibodies 1562 and 6H4 for study of PrPc expression on cultured endothelial cells (EC). Flow cyto-metry demonstrated that human umbilical vein ECs (HUVECs) and bovine aorta ECs are clearly positive for PrPc. We measured the number of PrPc molecules on HUVECs during growth in culture. The expression of PrPc peaked significantly (p<0.01) after 48 hours, when HUVECs were reaching confluence. Treatment of HUVECs with calcium ionophore led to shedding of PrPc positive microparticles. PrPc positive microparticles were also found in cell culture supernatant of non-stimulated HUVECs. Our results demo-nstrate that vascular ECs express PrPc and should be considered as a possible source of plasma PrPc and TSE infectivity.