Abstract.To increase purity and potency of the antihemophilic factor (AHF) concentrate prepared by the American Red Cross method, the following modifications were introduced: (1) A cold extraction step was incorporated to remove cold‐soluble impurities. The cryopre‐cipitate (cryo) was extracted with 0.02MTris buffer, pH 7.0 (4 ml/g cryo) at 0°C. Factor VIII loss in this step was negligible. (2) AHF was then recovered from the cold‐insoluble portion of the cryo by extraction at 21°C with the same buffer. To increase the AHF concentration, this second extraction step was carried out with a smaller buffer volume (2 ml instead of 3–4 ml/g cryo). The subsequent steps, deprothrombinization, filtration and lyophilization were essentially unchanged. To further increase factor VIII concentration, the dried AHF concentrate was reconstituted to 40 rather than 50% of the initial volume. AHF concentrate prepared on a large scale by this method was 20‐ to 30‐fold concentrated and 40‐ to 50‐fold purified over plasma at a recovery of about 250 factor VIII units per liter of plasma. The final product was readily soluble, clear and almost colorless up