Investigations were performed concerning the influence of the pH on the stability of factor VIII during the collection of blood and during the storage of blood and plasma for varying periods and at varying temperatures. It was found that the low pH of ACD anticoagulant solution (pH 4.9) caused a loss of factor VIII procoagulant activity of 10-15% during the collection of blood. However, when less acidic anticoagulant solutions were used, substantial losses of factor VIII occurred during the storage of blood. We concluded that the optimal pH of both the anticoagulant solution and the stored blood, should be between 6.7 and 7.0. However, no anticoagulant solution is known that meets these requirements. In practice ACD ensures the highest recovery of factor VIII in cryoprecipitate, at least in those cases where the blood donations are stored for several hours before separation and freezing of the plasma.