Prophylaxis of infection and alloimmunisation is the main reason for leucocyte depletion by filtration of blood components. The question is whether all red cell concentrates (RCC) should be filtered and whether plasma has to be filtered, too. For leucocyte-poor units whole blood was filtered before preparation using the 'top and bottom' system. These units of buffy-coat-poor RCC and plasma were compared with components filtered after preparation. Non-filtered RCCs and plasmas served as a control. By prefiltration of whole blood and filtration of components we obtained RCCs and plasmas with less than 1 x 10(5) leucocytes in every unit. In conclusion, leucocyte filtration before preparation seems to be an easy and cost-effective method in order to get two filtered components (RCC and plasma) with one filter.