Various counting methods have been described and reported for process control of leuco-epleted blood components. The recent production of high-efficiency leucocyte removal filters intensifies the need for sensitivity in determining the ever lower residual concentration of white cells (WBCs) in filtered units. In order to assess which method was the most efficient and feasible in the laboratory for the control of WBC-reduced packed red blood cells, we compared the sensitivity of four counting methods: Nageotte chamber analysis, flow cytometry, the fluorochrome method by Borzini and Nageotte chamber analysis as modified by Prati. We observed a difference in the post-filtration WBC content depending on which method of counting was used and we feel it reasonable to ask what method should be employed in blood component process control. The answer must naturally consider that the method is for use by a large number of laboratories, while the sensitivity of the method needs to be appropriate to the goal desired.