We investigated the possible role of platelet membrane vesicles on hemostatic function in vivo. Platelet membrane vesicles were prepared from rabbit platelets stored for up to 6 months at -65 degrees C and transfused into thrombocytopenic rabbits. Significant reductions in microvascular bleeding times were observed up to 24 hours after transfusion, with the greatest corrections at 4 hours. Measurements of factor V, factor VIII, fibrin degradation products, and fibrinogen in animals transfused with membrane ruled out intravascular coagulation and suggested a direct effect of platelet membrane vesicles at the bleeding sites. This conclusion was supported morphologically by identification of membrane vesicles in bleeding time lesions and radiologically by accumulation of 111In-labeled vesicles in lesions. Production of platelet membrane vesicles was simple, and freezing allowed long-term storage of a product capable of short-term hemostasis.