A fraction of FVIII:Ag in commercial recombinant FVIII (rFVIII) cannot bind VWF whereas all the FVIII:Ag in plasma‐derived FVIII (pd‐FVIII) concentrates does. To compare the FVIII:C activities of the fractions of rFVIII:Ag that can and cannot bind VWF. The FVIII:Ag contents of the rFVIII Kogenate, and Advate and a pd‐FVIII‐pd‐VWF (Fanhdi) were measured by ELISA. The FX activation was initiated by adding 1.0 IU of FVIII:C of each FVIII‐containing product to a coagulant phospholipids suspension containing 1.0 n
mFIXa, 100 n mFX, 1 μ mhirudin and 2 m mcalcium chloride and measured after 1, 5 and 10 min. The same approach was followed after adding 2.0 IU of pd‐VWF to1.0 IU of FVIII:C of Kogenate or Advate. The FVIII:Ag content/IU of FVIII:C of Kogenate, Advate and Fanhdi were 1.80 ± 0.05, 1.31 ± 0.9 and 0.84 ± 1.5 IU respectively. Only Kogenate and Advate effectively enhanced FX activation 1 min after adding each FVIII:C to the coagulant suspension containing FIXa and FX. Thus, the FXa initially generated by FIXa readily activated FVIII:C in control Kogenate and Advate to thereby effectively enhance FX activation while the VWF in Fanhdi continued to suppress FX activation for up to 10 min. Addition of pd‐VWF to Kogenate or Advate effectively decreased their enhancements of FX activation to the same level as Fanhdi over 10 min. The FVIII:Ag fraction in Kogenate and Advate that cannot bind VWF appears to be inactive as it has no measureable FVIII:C activity in the presence of added VWF in vitro.