Eighty units of whole blood in its third week of shelf-life were filtered through the Pall of Fenwal microaggregate filter. Half of these units were centrifuged before filtration. White cell removal was approximately 30% for the Fenwal filter both with and without prior centrifugation; for the Pall filter, removal was 7% without centrifugation and 30% with centrifugation. It is suggested that these simple filters are a useful way of reducing both platelet and white cell content of blood administered to patients requiring multiple transfusion. Their use may delay development of white cell and platelet antibodies obviating the use of washed or frozen cells in these patients.