Hemophilia B mice, that had been infused with 150 IU/kg BeneFIX 7 days previously, were subjected to the saphenous vein injury. After the vein ceased clotting, the contralateral saphenous vein was transected. The data show that there is no significant difference between the 2 veins in the number of clots formed, despite undetectable levels of circulating FIX at the time of the injury of the first vein. This indicates that it is not residual circulating FIX that is responsible for clotting, but instead extravascular FIX.