Although patients with severe hemophilia B have lifelong spontaneous hemorrhage and crippling hemarthroses, patients with 0.01–0.03 U/ml Factor IX activity have a milder disease state. The clinical condition of severely affected individuals could potentially be improved by prolonging the half-life of transfused Factor IX. The feasibility of incorporating Factor IX into red cell ghosts was suggested by resealing experiments with similar sized molecules such as albumin. We have prepared resealed red cell ghosts containing human Factor IX and X. Human red cells were subjected to hypotonic lysis at 0°C, pH 6.0. Commercial prothrombin complex concentrate was dissolved in the lysing medium immediately prior to the addition of the red cells. After being returned to isotonicity, the red cell ghosts were annealed at 37°C for 30 minutes and then washed extensively. When intact red cell ghosts were tested, no Factor IX or X activity could be demonstrated. After disruption of the red cell ghost membranes with 3M urea or 2% Triton X–100, the procoagulants could be quantitatively recovered. Similar recovery of the clotting factors could be demonstrated from the lysate and the early wash samples. Red cells from Factor IX and X deficient patients served equally well as those from normal subjects. Red cell ghosts prepared in similar fashion but not exposed to the procoagulants had negligible clotting activity. We have demonstrated that human clotting factors can be incorporated into red cell ghosts. The ability of this system to prolong the biological half-life of Factor IX is under investigation.