To simplify the quality control procedure used to determine the efficiency and consistency of bedside leukoreduction, a counting protocol using prefiltration and postfiltration aliquots fixed in 10% formaldehyde was designed. To assess the reliability of the values obtained by counting the formalin-fixed samples, a parallel study was performed using our standard protocol of counting fresh propidium iodide-stained samples in a Nageotte chamber under a fluorescent microscope. A total of 30 single-donor platelet concentrates and 30 units of packed red blood cells were analyzed in parallel using the standard and formalin-fixation methods. Furthermore, the aliquots fixed in formaldehyde were split and counted at 1, 3, and 30 days. The results showed no significant quantitative difference between the two methods. Of note is that the counts in formaldehyde-fixed samples at 1, 3, and 30 days were consistent among themselves. The formaldehyde fixation of samples obtained for quality control of leukoreduction allows blood collection and storage at 4 degrees C and batch counting when and where convenient.