SummaryA method for obtaining concentrates of platelet cofactor I (factor VIII) from ether extracted bovine serum has been developed. Platelet cofactor I is inactivated during the coagulation process by undergoing a complex formation, which is broken by ether extraction.In each of 13 preparations from 1,000 ml of extracted serum an average total activity of 3,055 platelet cofactor I units was isolated. The specific activity was 4,760 units per milligram tyrosine and 138 units per milligram protein (Biuret.)In the paper electrophoresis the fractions were homogeneous. Using immunoelectrophoretic techniques three bands were found against anti-bovine serum from rabbits. The preparations activated purified prothrombin in the presence of platelets, Ac-globulin and calcium ions. They substituted in the thromboplastin generation test for adsorbed plasma and corrected the delayed thromboplastin generation of hemophilia A plasma. They also corrected the prolonged recalcification times of hemophilia A plasma. The preparations lost their activity upon storage at room temperature and in the icebox rapidly. Also freeze drying resulted in a complete loss of activity.