Abstract: Highly purified, biologically active Human Factor VIII:C having specific activities of about 4000-8000 units per milligram of protein is prepared. In the method of preparation, an AHF concentrate is solubilized or equilibrated in an aqueous medium and treated to change the effective Stokes' radius of the Factor VIII:C to an apparently low value and then subjected to a separation from the concentrate. Treatment of the highly purified Factor VIII:C with a mixture of glycosidases causes substantial removal of carbohydrate side chains without reduction of procoagulant activity and with retention of significant in vivo survival time.