Summary:Patients receiving multiple whole blood transfusions often experience adverse clinical Symptoms caused by leukocytes. In the present study, the efficacy and safety of a polyester filter for leukocytes (Sepacell R-500) was evaluated. Donor units of whole blood and red cell concentrate stored for varying lengths of time were filtered. Emphasis was placed on humoral parameters indicative of release and/or activation reactions of granulocytes (neutral proteinase elastase, lysozyme, aggregation, chemiluminescence) and erythrocytes l (lactate dehydrogenase, plasma haemoglobin). Nonspecific adsorption effects were investigated by plasma , .protein determinations (albumin, a 2 -macroglobulin). A complete blood cell count äs well äs values of 1 haemoglobin and haematocrit were determined. Sepacell R-500 proved to be a highly efficient filter to the j leukocyte depletion of blood. Our results of erythrocyte and granulocyte related humoral parameters provided no significant evidence of filtration mediated activation or releasing reactions of clinical consequence.