BACKGROUND: Recently, sufficient numbers of lymphocytes that may cause alloimmunization have been detected in fresh frozen plasma (FFP) and the need for filtration and/or irradiation of FFP for certain patients has been proposed. We examined the numbers of white blood cells (WBCs) in FFP and evaluated a new WBC-reduction filter designed for FFP. METHODS: A total of 118 units of FFP were analyzed. We counted WBCs by using the crystal violet-staining method. Forty-four units of FFP were tested for WBC numbers before and after filtration by using the Nageotte chamber. The viable WBC count was performed using the 0.4% trypan blue-staining method. The biologic activity of the coagulation factor viii was measured in 34 units of FFP before and after filtration with the ACL 3000 (Instrumentation Laboratory, Milano, Italy). RESULTS: In the 118 units of FFP, 39 units (33.1%) and 1 unit (0.9%) had counts greater than 1X10(6)/unit and 5X10(6)/unit, respectively. The filter reduced the WBC numbers to less than 1X106/unit in all units. The average percentage of viable WBCs before and after filtration was 34.9% and 4.7%, respectively. The average leukocyte removal rate of the WBC-reduction filter was 81.0%. The average factor viii activity before and after filtration was 72.2 U/dL and 75.1 U/dL, respectively. CONCLUSIONS: Some FFP prepared by the Korean Red Cross blood center may contain enough to require WBC-reduction filtration. Therefore, WBC-reduction filtration of FFP should be considered in order to prevent adverse effects, particularly in those patients expected to receive repeated transfusions.