Several studies have suggested that the efficacy of leukocyte (white blood cell, WBC) removal by filtration is affected by storage time, temperature, and rate of filtration. However, these studies did not clearly differentiate among storage, interdonor, and interfilter variables. This study was designed to determine the influence of time, temperature, and expression of leukocyte adhesion molecules on the efficacy of leukoreduction of red cell units in a controlled format. Ten donors underwent whole blood phlebotomy on each of three occasions. Units underwent filtration using a standard, commercially available RCXL-1 filter (Pall Corp.) as follows: (1) pre-storage filtration at 22 degrees C using laboratory conditions and gravity flow rates; (2) filtration after 14 days of storage at 4 degrees C, using laboratory conditions and gravity flow rates; and (3) filtration after 14 days of storage at 4 degrees C, using mock bedside conditions. Mock bedside conditions consisted of flow through an IMED electromechanical pump at 22 degrees C at a rate that would infuse the product over 2 hours. Pre- and post-filtration red and white cell counts and leukocyte CD 11a expression were assessed on Days 0 and 14. Post filtration white cell counts were determined by propidium iodide staining and Neubauer chamber counting. WBC content pre- and post-filtration was lower in the two post storage groups (residual WBCs 1:02 x 104 for in-laboratory vs. 2.31 x 104 for bedside filtration) than in the pre-storage group (52.8 x 104 residual WBCs), and log 10 reduction in WBCs was significantly greater in the two post-storage vs. pre-storage groups (4.59 vs. 3.83 log 10 reduction). Mean post-filtration RBC recovery was greater than 80% with all three filtration techniques. Leukocyte expression of LFA-1 as measured by CD11a was similar in all three groups. Our study demonstrates that the efficacy of leukoreduction is significantly greater in red cell units undergoing post-storage rather than pre-storage RCXL-1 filtration. In post-storage filtered units, neither the temperature nor rate at which filtration was performed affected the degree of leukodepletion, although a greater range of residual WBCs was seen with bedside filtration. These findings suggest that optimal leukodepletion of red cell units is accomplished in the laboratory rather than at the bedside, and that leukodepletion is least effective if filtration is performed immediately after the units are collected.