A device is described which can be attached to all blood bags. Two of these devices are heat-sealed together aseptically at any time during the storage life of the blood unit, and an opening connecting the two is created. In vitro and in vivo studies show that ten passes of a unit of blood through these devices produces no cytological changes and does not alter the in vivo survival of the unit when transfused. Further, if the unit is frozen, then thawed, and even stored at 4 C for five days after thawing, the in vivo survival is still unaffected.