BACKGROUND: White cell reduction of blood products minimizes the risks of alloimmunization against HLA-antigens, the transmission of viral diseases and the incidence of platelet transfusion reactions. One modern strategy is leukocyte depletion with an integrated filter system immediately after preparation and prior to storage. STUDY DESIGN AND METHODS: We evaluated the efficiency of a novel in-line filter system Sepacell PLX-5 BPS for leukocyte reduction of platelet concentrates (PC) from pooled buffy-coats. A total of 44 PCs were investigated with regard to different filtration flow rates (25-110 ml/min) and leukocyte depletion and thrombocyte recovery rates were analysed. Furthermore, we studied the influence of filtration on PCs over a storage period of 6 days (n = 12) by investigation of pH, lactate and glucose. Platelet function was determined by means of hypotonic shock response, external shape change and expression of CD62p. RESULTS: The mean leukocyte depletion rate was > log 5. After filtration the mean leukocyte count was 0.12 +/- 0.21 x 10(6). In 60% of the PCs the leukocyte count lay below the detection level of the Nageotte chamber, which is < 0.3 x 10(5). The flow rate correlates significantly with the leukocyte count in the PCs (r = 0.325; p = 0.033) and therefore with the leukocyte depletion rate (r = -0.422; p = 0.01). Flow rates under 40 ml lead to a significantly lower leukocyte contamination. Only in one PC, at a flow rate of 84 ml/min, was the leukocyte threshold of 1 x 10(6) exceeded. We did not find a significant correlation between filtration flow rate and thrombocyte recovery (r = 0.315; p = 0.069). The mean platelet count in the PC was 2.88 +/- 0.47 x 10(11). Compared with the thrombocyte count in the pooled buffy coat, the recovery was 68.6%. We observed a decrease of pH, glucose, external shape change and hypotonic shock response over the storage period while lactate and the expression of CD62p increased. CONCLUSION: The filter system Sepacell PLX-5 BPS proved to be suitable for in-line filtration of platelet concentrates prior to storage. Filtration flow rates of up to 40 ml/min allowed efficient leukocyte depletion without significant loss in the quality of the platelet concentrates and the platelet function in vitro.