sFlt1 is central to the pathophysiology of pre-eclampsia. Excitingly, in 2008-09 three groups reported the predominant placental-derived sFlt-1 isoform is a newly identified splice variant named sFlt1-e15a. Given this is the main sFlt-1 isoform released from placenta, it may be the key pathophysiological factor of pre-eclampsia. To date, the protein expression of sFlt1-e15a has not been described and there have been no functional studies of sFlt1-e15a. We aimed to characterize sFlt1-e15a in preterm and preeclamptic placenta and examine whether sFlt1-e15a protein is biologically active.To investigate expression of sFlt1-e15a we developed polyclonal antibodies targeting a unique peptide sequence in the C-terminal region of sFlt1-e15a. To test whether sFlt1-e15a neutralizes VEGF activity, we used a novel bioassay; BAF3 cells engineered so that they require VEGF to survive. We next assessed whether sFlt1-e15a blocks VEGF induced migration and invasion using the xCELLigence system (monitors experiments in real time), and finally assessed whether sFlt-1-e15a blocked VEGF induced signaling by Western Blot.The specificity of our antibodies was established by western analysis. Immunofluorescence confirmed expression of sFlt1-e15a principally within the syncytiotrophoblast layer of placenta, with expression appearing increased in pre-eclampsia.Recombinant sFlt1-e15a dose dependently reduced BAF3 cell viability in the presence of VEGF, suggesting it directly antagonizes VEGF and is biologically active. In the presence of VEGF, sFlt1-e15a also potently blocked primary HUVEC migration and invasion. We next assessed whether sFlt1-e15a blocked VEGF induced signaling of Akt by exposing primary HUVECs to VEGF ± sFlt1-e15a. VEGF induced phosphorylation of Akt (normalized to total Akt) was potently blocked by sFlt1-e15a.The novel primate and placental specific sFlt1-e15a protein is present in pre-eclamptic placenta, biologically active and neutralizes VEGF. Being the main sFlt1 splice variant expressed in preeclampsia, sFlt1-e15a may be the main sFlt-1 isoform responsible for the pathophysiology of preeclampsia.T. Kaitu’u-Lino: None. K. Palmer: None. R. Hastie: None. P. Cannon: None. N.J. Hannan: None. S. Tong: None.