The present invention relates to a proteinaceous construct comprising a blood coagulation factor, e.g., Factor VIII (FVIII), being bound to at least one water soluble polymer, including a poly(alkylene oxide) such as polyethylene glycol (PEG). Further the present invention relates to methods of preparing PEGylated blood coagulation factor, e.g., FVIII, in the presence of bound antibodies. The invention also relates to methods for prolonging the in vivo-half-life of blood coagulation factor, e.g., FVIII, in the blood of a mammal having a bleeding disorder associated with functional defects or deficiencies of blood coagulation factor, e.g., FVIII.
