To determine the actual efficacy of red cells filtration technique, the "Labile Blood Component Production" french study group, including 21 blood centers, realized a large study on more than 1,400 filtrations and 3,000 controls. 745 units of red cell concentrates (RCC) and 690 units of buffy-coat poor red blood cells (BC PRBC) were filtered through 6 commercialized leukocytes depleting filters: not less than 170 experiments per filter, tested by 3 different blood centers. Pre-filtration controls prove that buffy-coat removal, done manually or with automated equipment, involves a first leukocytes depletion about 63% and an hemoglobin loss equal to 4 g (7%). After filtration, residual leukocytes counts were performed manually in a Nageotte counting chamber. In this study, we evaluated the reliability of this simple method which accurately measures very low leukocytes counts. The variation coefficient was 25% for 2.5 leukocytes/microliter concentration (O.6 x 10(6) per filtered unit). The results, obtained from 1200 evaluated filtrations, confirm that buffy-coat removal obviously improves the filtration performances (residual leukocytes level is lower than 1 x 10(6) per unit for 78% filtered BC PRBC versus 43% filtered RCC). Furthermore, 90% of overall filtered units are containing less than 5 x 10(6) leukocytes.