The exact number of leukocytes remaining in 'leukocyte-free' red cell and platelet concentrates cannot be measured by standard methods. We have therefore developed methods to harvest all the leukocytes from blood components. The leukocytes were then counted and identified using monoclonal antibodies. The leukocyte-free red cell concentrates were prepared by combining buffy coat removal and filtration through a Cellselect filter. The mean number of leukocytes per unit was 1.0 X 10(5). Most of the leukocytes were granulocytes and T cells. Only a few B cells or monocytes could be detected. Leukocyte-free platelets were prepared by filtering 4 units of PC through a cotton-wool (Imugard) filter. The mean number of leukocytes per PC unit was 0.4 X 10(5) of which 85-95% were T cells.