BACKGROUND White cell filtration of red cell concentrates is often performed at the bedside, in the ward, with the filter inserted in the blood administration line. The aim of this study was to evaluate the efficiency of this filtration method and compare it to filtration in the blood bank. METHODS One-day-old, buffy coat-reduced, hard-packed red cell concentrates in saline-adenine-glucose-mannitol solution were filtered through different filters designed for bedside or laboratory use. With filters designed for bedside use, filtration of red cells was performed under laboratory conditions at fast flow (10 min) or under bedside conditions at slow flow (2 hours). The remaining white cells were counted microscopically. Filters designed for laboratory use were evaluated at fast flow, and the number of contaminating white cells was counted by flow cytometry. RESULTS With bedside filters, a significantly higher contamination of white cells was found in the units filtered at slow flow than at fast flow, regardless of the filter used. The number of units with > 5 x 10(6) white cells was 52 (78%) of 67 filtered at slow flow compared to 11 (23%) of 47 at fast flow, all filters taken together. This difference in white cell contamination was mainly due to an increase of polymorphonuclear cells in the red cell concentrates filtered at slow flow. With filters designed for laboratory use, 0 to 2 percent of units (n = 1448) were contaminated with > 5 x 10(6) white cells. CONCLUSIONS Bedside filtration for white cell reduction at slow flow is inefficient for 1-day-old, buffy coat-reduced red cell concentrates.