A new method of filtering marrow obtained from marrow donors during marrow transplantation is described. The technique involves a small reusable filter unit and provides a simple, inexpensive, and fast means of filtering marrow in a predominantly closed system prior to infusion into the marrow recipient. In vitro studies of marrow from 10 donors showed no reduction in granulocytemacrophage or erythroid progenitors following filtration. Clinical studies of 75 patients receiving marrow filtered by this system showed no untoward effects or interference with engraftment. We conclude that this predominantly closed filtration system is efficient and should be further evaluated in marrow transplantation.