Three different properties of factor VIII (antihemophilic globulin) were studied after infusion of 500 ml of six per cent Dextran 70 to healthy volunteers. This dose caused no change in the factor VIII coagulant activity while there was a significant but temporary decrease of factor VIII related antigen. The Ristocetin induced platelet aggregation analysed with native as well as with formalin-fixed platelets, also decreased significantly. The maximal decrease was found a few hours after the infusion. The decrease was seen only if Dextran was infused in vivo and not if Dextran was added to blood in vitro. The dysfunction of factor VIII caused by Dextran resembles that found in one variant of von Willebrand's disease. This finding probably explains the platelet function inhibiting properties of Dextran, and may be of significance for its antithrombotic effects.