A procedure involving cryoprecipitation of human plasma in the presence of polyethylene glycol results in consistently higher yields of antihemophilic factor and von Willebrand factor than were heretofore achieved by cryoprecipitation alone. The resulting AHF and vWF are at least as pure as ordinary cryoprecipitate and can easily be further purified by additional processing. The method is expected to be especially useful in the preparation of AHF and vWF concentrates for the treatment of patients with hemophilia or von Willebrand's disease.