SummaryProtein S (PS) is an anticoagulant plasma protein whose deficiency is associated with increased risk of venous thrombosis. PS directly inhibits thrombin generation by the blood coagulation pathways by several mechanisms, including by binding coagulation factors (F) Va and Xa. To identify PS sequences that mediate inhibition of FVa activity, antibodies and synthetic peptides based on PS sequence were prepared and employed in plasma coagulation assays, purified component prothrombinase assays, binding assays, and immunoblots. In the absence of activated protein C, monoclonal antibody (Mab) S4 shortened FXa-induced clotting in normal plasma but not in PS-depleted plasma. Mab S4 also blocked PS inhibition of FVa-dependent prothrombinase activity in purified component assays in the absence or presence of phospholipids and inhibited binding of PS to immobilised FVa. Epitope mapping identified N-terminal region residues 37–67 of PS as this antibody’s epitope. A peptide representing PS residues 37–50 inhibited FVa-dependent prothrombinase activity in a noncompetitive manner, with 50% inhibition observed at 11 µM peptide, whereas a peptide with a D-amino acid sequence of 37–50 was ineffective. FVa, but not FXa, bound specifically to the immobilised peptide representing residues 37–50, and the peptide inhibited binding of FVa to immobilised PS. These data implicate PS residues 37–50 as a binding site for FVa that mediates, at least in part, the direct inhibition of FVa-dependent procoagulant activity by PS.