A newly developed filter-system (Sepacell PL) for the removal of leukocytes from single donor platelet concentrates was tested. It removes contaminating leukocytes by 99.9%, the platelet loss was calculated as less than 1%. Platelet ultrastructure and function do not seem to be altered by the filtration. Therefore, the tested filter system seems to be a suitable device for leukocyte depletion of single donor platelet concentrates. Further clinical studies, however, are necessary to prove the efficiency of the filter system in preventing alloimmunization in vivo.