Flow cytometry was used to: (1) determine residual leucocyte numbers in red cell suspensions following the range of leucocyte depletion procedures used in our organisation, and (2) to characterise phenotypically the leucocytes using direct immuofluorescence with monoclonal antibodies to cell surface receptors. Under the conditions used, a lower limit of detection of 2.5 leucocytes per µl (equivalent to 3.43 log(10) or 99.96% removal) could be achieved. Filtration through polyester filters was found to remove up to >99.96% of the initial leucocytes; however, a significant differential efficacy was observed between filters from different manufacturers even when filters with similar costs were compared. The order of filter brands with respect to leucocyte removal found was Pall BPF4 = Erypur Optima G-O > Sepacell R500 > Pall RC50. Pheno- typing revealed that increasing filtration efficacy was associated with a preferential removal of lymphocytes; conversely, a second filtration over one brand of filter allowed proportionately more lymphocytes to pass through compared with the first filtration. A saline wash following filtration removed a further 0.5% of the initial leucocyte content, and was associated with a preferential loss of granulocytes. Freeze-thawing the red cell suspension removed fewer leucocytes (96.3%) than did filtration (98.74% to >99.6%) or filtration followed by washing (99.22%), and also led to preferential loss of granulocytes. Flow cytometry provides a reliable tool for the quality control of leuco-depleted red cells, and allows a qualitative assessment of the residual leucocytes. This information is of value in choosing procedures aimed at decreasing the risk of alloimmunisation and post-transfusion reactions.