Transplantation experiments were utilized to study the possible sites of synthesis of von Willebrand factor (vWF) and factor VIII (F VIII) activities. Three normal kidney and two normal liver allografts were implanted into five swine with von Willebrand's disease (vWD) that survived for 1,6, and 7, and 4 and 9 days, respectively. The correction of the multiple hemostatic defects of vWD by organ transplantation was evaluated using the F VIII procoagulant activity, bleeding time, and platelet aggregating factor (PAF) levels; i.e., vWF levels. Normal kidney allografts produced no changes in the bleeding times or increases in F VIII or PAF. Transfusions for surgical hemostasis produced transient increases in F VIII and PAF. In animals receiving normal liver allografts, the levels of F VIII exceeded 100%, PAF was increased, and sustained correction of the bleeding time and maintenance of hemostasis was observed. These data suggest that the kidney is incapable of synthesizing either the vWF or the F VIII and that cells contained in the liver, possibly the endothelial cells, are one of the sites of synthesis of these factors.