There is almost general agreement that removal of leukocytes from blood components reduces the incidence of HLA-antibody formation and refractoriness to random platelet transfusions. Recently filters have become available, which are able to reduce leukocyte contamination in platelet suspensions with acceptable platelet loss. We evaluated a cellulose acetate (CA) and a polyester (PE) filter, and stored huffy coat-derived platelet suspensions after filtration. Both filters are effective for the removal of leukocytes to levels below 5 x 10^6 per transfusate. For the CA filter, platelet recovery was 73±13% yielding 256±53 x 10^9 platelets per transfusate from 6 donors. For the PE filter, platelet recovery was 90±9% and 327±51x 10^9 platelets per transfusate. When a loading dose of less than 5 x 10^8 leukocytes was applied, 98% of the CA-filtered suspensions and 100% of the PE-filtered suspensions contained less than 5 x 10^6 residual leucocytes. In 123 patients transfusion results of CA-filtered platelet suspensions stored for 72 h, were compared with those obtained by non-stored, non filtered, random platelet suspensions which had been leukocyte depleted by differential centrifugation. Platelet increments 1 and 20 h after transfusion showed no statistical difference between CA-filtered platelet transfusions stored for 72 h and non-stored. non-filtered platelet transfusions. In a new cohort of 117 patients, two filters and various postfiltration storage times were compared. Using both filters, the 1-hour posttransfusion increments decreased to approximately 60% after 96 h of storage compared to results of storage periods of 72 h or less. Moreover, gradually decreasing platelet survical, as measured from 20-hour post-transfusion increments, was observed with increased storage time, from 24 h onwards. This decrease in platelet survival was associated with a decrease in pH. Because the pH and storage interval are strongly related, pH could not be evaluated as an independent parameter. The storage-related decreased platelet survival, however, was more severe after PE filtration than after CA filtration, whereas pH was better maintained after PE filtration. In 164 cases the bleeding time was measured after transfusion of platelet stored for ≥72 h. Provided that a posttransfusion increment of ≥15 x 10^9/l was reached after transfusion with filtered, stored platelets, bleeding times shortened, indicating the hemostatic activity of filtered and subsequently stored platelets.