AbstractActivated protein C (APC) resistance is a major risk factor for venous thrombosis. Factor V (FV) gene mutations like FVLeiden (R506Q) and FVR2 (H1299R) may cause APC resistance either by reducing the susceptibility of FVa to APC-mediated inactivation or by interfering with the cofactor activity of FV in APC-catalyzed FVIIIa inactivation. We quantified the APC cofactor activity expressed by FVLeiden and FVR2 and determined the relative contributions of reduced susceptibility and impaired APC cofactor activity to the APC resistance associated with these mutations. Plasmas containing varying concentrations of normal FV, FVLeiden, or FVR2 were assayed with an APC resistance assay that specifically measures the APC cofactor activity of FV in FVIIIa inactivation, and with the activated partial thromboplastin time (aPTT)-based assay, which probes both the susceptibility and APC cofactor components. FVR2 expressed 73% of the APC cofactor activity of normal FV, whereas FVLeiden exhibited no cofactor activity in FVIIIa inactivation. Poor susceptibility to APC and impaired APC cofactor activity contributed equally to FVLeiden-associated APC resistance, whereas FVR2-associated APC resistance was entirely due to the reduced APC cofactor activity of FVR2. Thrombin generation assays confirmed the importance of the anticoagulant activity of FV and indicated that FVLeiden homozygotes are exposed to a higher thrombotic risk than heterozygotes because their plasma lacks normal FV acting as an anticoagulant protein.