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A recombinant Factor VIII (Factor VIII-delta II) consists of a unique polypeptide chain of 165 kDa deleted from the major part of the B-domain and from the cleavage site at Arg-1648-Glu-1649 found in plasma-derived Factor VIII. It was expressed in mammalian cells in serum-free medium containing von Willebrand factor and purified by a one-step immunopurification. The recombinant Factor VIII was characterized as a single active peak when subjected to f.p.l.c., in contrast with the plasma-derived molecule. Its coagulant activity was decreased in the presence of EDTA, suggesting that a bivalent ion is required, as for plasma-derived Factor VIII. The activation by thrombin and the inactivation by activated protein C were studied and the resulting molecular forms were analysed by f.p.l.c. and SDS/PAGE. The results clearly demonstrate that, despite the structural differences between plasma-derived and recombinant Factor VIII, activation and inactivation of Factor VIII-delta II generate proteolysed complexes similar to that described for plasma-derived Factor VIII. Thus this deleted recombinant Factor VIII, which is processed similarly to plasma-derived Factor VIII, should be normally integrated in the regulation system of Factor X activation in the blood-coagulation cascade.